Theme Font & Size Changer for Thunderbird (#E6) [update 64.3 #2]

[You’re late to the party. This crap has been fixed and is now available as addon directly from Mozilla. Click here for the addon announcement, or read the comments on this page for a secondary fix (64.4) for TB58+.]

You know what really grinds my gears? MORONS that “maintain” a piece of software.

This guy called Baris Derin has quite a lot of Mozilla (FF/TB) addons out there. Great stuff. I’ve been using the font changer for quite some time, as my Thunderbird looks a bit odd with default settings. So, what’s wrong with it?

This addon fails every so often.

No, it’s not Thunderbird updating and adding weird stuff or changing some of the API. I know when my apt says “there’s an update for thunderbird”. And today, there was none. Same yesterday. And the friggen addon failed ONCE AGAIN for no valid reason.

Well, the commentators on do know why, even if most of the helpful posts get deleted for some reason (I don’t think Mozilla deletes them just for fun, but that’s just my point of view…).

This IDIOT is using the following lines of code in bootstrap.js:

const build = 1515509396;
(line 3)

if((build+'').length != 10) Services.prompt.alert(null, 'CompileAddon', 'An error occured');
if(new Date().getTime() > ((build+'').length == 10 ? build*1000 : build)+(3*30*24*60*60*1000)) return false;
return true;

(lines 1380ff)


1515509396 is obviously an unix time stamp. Built/updated on Tuesday, 09-Jan-2018 14:49:56 UTC to be precise. Today is April 9th 2018. Guess what + 3*30*24*60*60 (*1000 for milliseconds) does – it adds three months (well, 90 days) of lifetime to the build date. If that number is smaller than the current timestamp from new Date().getTime() – bingo – it returns false. Addon silently fails. No warning message, no errors in the log, no nothing. The addon silently fails after three months if the author does not push a new version. And the last 3 times he did, his “new” version was the very same source code with a different const build value.

Sir, you are an idiot.
Go get a job at Oracle and GTFO of the open source world.

Here’s a “fixed” version of the current v64.1 (theme_font_size_changer_for_tb-64.1-sm+tb.xpi):
(rename to .xpi instead of, WordPress doesn’t allow .xpi uploads)

(extract once and the .xpi file is right there – no need to enable file extension display in your file manager)

Update: Here’s a fixed version for Thunderbird and also for older Firefox versions up to 56.x, because for 57+ (“Quantum”, wank, wank) they’ve changed UI crap and finally render XUL extensions like this unusable. I’ve re-added FF compatibility and zipped the file for your convenience – just unzip and add to TB/FF (see comments if you need help)


Also, it only took five days for Mozilla to “approve” this as a new addon. As the first revision was indeed buggy, I’ve just submitted the 64.3 version. Expect it to work around next week or so [edit: it took THIRTEEN DAYS] – once it is hosted at Mozilla, you can install it just like every other addon. This also fixes the issue if Baris Derin decides to push a new update – this fixed version will not update back to his tree.

This is the very same source code except the if(new Date().getTime() >… line is removed. One could also change the build date to something in the distant future, but don’t exceed 10 digits, as the first line checks that as well. It’ll throw an error, though…

If that does solve your problem with this otherwise fine addon – let me know and report the addon as not meeting the addon policy:

Should an add-on include any unexpected feature that falls into one of the following categories:


Potentially compromises user privacy or security (like sending data to third parties)
Changes default settings like the new tab page, homepage or search engine
Makes unexpected changes to the browser or web content
Includes features or functionality not related to the add-on’s core function(s)


Then the “unexpected” feature(s) must adhere to all of the following requirements:


The add-on description must clearly state what changes the add-on makes.
All changes must be “opt-in”, meaning the user has to take non-default action to enact the change. Changes that prompt users via the permissions system don’t require an additional opt-in.
The opt-in interface must clearly state the name of the add-on requesting the change.

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Module stopped working today.

Thanks for the updated module !




Bryan Allen

My hat is off to you, sir!
Excellent work!

Thomas Blasejewicz

Good evening
I downloaded your fixed version and it actually DID change (solve) my problem. At least for now.
The addon is now working – which it did not, when I started my computer TODAY.


Hello, thanks so much for this – it works !


Claudio Guglieri

I had this problem but after the fix now it’s allright. Thanks for your work.



Thank you so much!


Please excuse my ignorance. How do I install this “fixed” version. ?


Not working for my Mac–corrupted file message. Any thoughts?


I’m still having trouble with this. I have changed file extension to.xpi. But when pointing to the file from thunderbird “install Add-on from file” I get a series of three folders and three other files. I am not sure what to click to install the update/addon


I have the same question. Using Mac OS 01.11.6 and Thunderbird 52.7. I get a corrupted file message when I click on the zipped folder, or one the folder, or on anything within that folder.


that would be great–thanks.


Done! Hallelujah! Thank you, sincerely.


This worked! Thank you so much!


Sorry. Idiot here. Do I drop the “.zip” from the downloaded zip file? I tried that but still don’t get a file that is “add-ons” format that is recognized when I click “install add-on from file”. There are only several folders with greyed out files. Desperate to get this fix. Appreciate any help. Yeah, I know – any ‘remedial’ help!

Kenny Camp

Getting “corrupt” message and have another “should know this but don’t” question. Do you open the Zip file, and click on the folder? Or Open the folder and click on the install icon? I changed the file extension on both the zip file and the folder. None of these things seems to prevent the “corrupt” message from appearing. Help appreciated!

Mark B.

Thanks for creating/posting this update. My eyes thank you 8^)


You are amazing thank you for fixing the add-on!!!!!!! I also enjoyed your commentary regarding the dev who made the add-on, LOL. =)


The original developer seems to have disappeared. I sent him polite 2 emails to which he never replied. Very strange, TFSC is a great add-on. Why put the 90-day ‘time bomb’ in it and why seemingly abandon it?


Thanks for this.


>>Well, the commentators on do know why, even if most of the helpful posts get deleted for some reason <<

I could not figure out why I no longer see my posting there.
I basically told others to do the same – edit the code to remove this pesky time stamp.
People thanked me but my posting was nowhere to find :cry:


Thank you very, very much! Used the second link for Windows 7, unzipped it and installed it, works fine. :yahoo:

Any chance you would be willing to do the same for the browser version so we could use it in Waterfox?

I would be willing to make a donation for these fixes. They make a big part of life far less annoying since reading glasses combined with my computer screen causes eye pain.

Wishing you the best in life, take care.


I’m using Mac and you Sir are a legend – thanks so much for this fix


Incredible. I can’t believe I finally found your FIXED version. I have used it for years and never understood why it stopped working all of the time. Kudos Bzzz, you are the BOM B-)


Can somebody help me. I installed this, but I can’t seem to find the settings “button”?!
I’m running Thunderbird 60.0b2 (32-bit)

Regards, Smakodak


Thank you. I tried setting the extensions.strictCompatibility to false, but that didn’t do it.
For now i’ll just have to pinch my eyes :shock:


It does. Thank you :yahoo:

Maya H

Downloading code from an unknown source (especially one w/ a wicked sense of humor such as WHL) made me paranoid – but I couldn’t stand living w/out TFSC anymore (after the infamous Apr 9 update caused it to magically vanish), so brought my MacBook to my friendly Mac repair guy to read WHL’s code & give his approval. He gave. I downloaded. Success! THANK YOU WAN HUNG LO God/goddes of open source!!!!!!!!!!

Grateful One

Thank you for this fix; it’s a godsend for my eyesight and my sanity. Works great on mac and windows with none of the issues of the addon it’s fixing. Glad to hear yours will be part of the addons for TB in the near future so I can stop emailing Baris Derin on a daily basis to say their addon stopped working again and the supposed fix or update they’ve posted doesn’t work either.

FYI I found the link to your site on a fluke here where you posted it as a review for this addon by Baris Derin:

What’s odd is that that review showing your web address isn’t shown in the reviews at all if you access them thru Thunderbird’s Addons button in the toolbar of the app, then click on Get Addons (in the Addons Manager tab) and finding the Theme Font & Size Changer for ThunderBird 64.1 addon and its reviews. So folk there are missing out on coming here and finding your fix for the addon. You may want to post or re-post there so all those complaining in the current addons reviews on there know you have a viable and working alternative and one that will be an addon in the near future.

Thank you again for fixing this issue and for sparing me eyestrain, frustration and headaches; it’s an invaluable tool many people need and rely on. You’re awesome! :good: :good: :good: :heart:


Are you also developing a TF&SC for Firefox 57+ and Thunderbird 60+ ?

Dr. John Meyer

Thank you so very much. And my opthalmologist as well.

Grateful One

Thank you for this fix. It’s a life and sight saver. I hope everyone that needs it finds their way here to get the file.

Shaul k

Many thanks. This works


Question about your addon. When it’s listed in the addons for Thunderbird, will it have the same name or be called something else so it’s not confused with the Baris Derin one?


Success! Back to Verdana Font size 16. BTW, it appears that the TB default font size = 6. WTF?



I can’t get your add-on preferences window to show up (where I can modify fonts etc) in TB 60.0b4 (64-bit)

I set extensions.strictCompatibility to false as suggested above – any ideas?




works like a charm! thank you! :good: :good: :good:


Just want to add my thanks for the fix. As many have said, Thunderbird is really not usable for many without this. Mozilla’s default font size (6?) is just interface malpractice…. Do they offer a rationale for that and the lack of easy control of the sizing?

[…] [Original post is here] […]


Super-dense in Hollywood……

Hi Bzzz, I have TB 45.8.0, and don’t want to upgrade as would also have to upgrade Mac OSX 10.6.8, which don’t want to upgrade for even more reasons.

Are these not going to work with your fix ?

I downloaded the folder at your first link, but in trying to install, all the files are dimmed.

Then I caught your comment “Permanently changing allowed file types in WordPress is a nightmare, so I decided for the easy alternative: Zip file of the .xpi is now online. Please have a try.”

Online where ?

It’s breathtaking to see the followup work you’re doing on this. Hard to believe this snafu has persisted so long.

Alan Little

Harrarramar. Yes did indeed try the fixed version (after deleting the problematic version on TB). Installed ok, but hangs up TB when I click on settings.

However, will give it a go again. Momentary !

Quantum Leap Unlimited

Onward, Upward

Alan Little

Ooooook. Same result as before.

1. 64.3 installed successfully

2. Quit TB

3 Restart

4. Click on change preferences icon: 3 things displayed……
hide tools menu, hide app menu, abbreviate toolbar. I click none of the 3.

5. TB is now frozen, restart necessary


60.0b6 – I have the same. Not work.

Alan Little

Hi Bzzz, yeah, TB 45 was working fine with theme font until (like everyone else) they did the screwy update.

What are the chances the original developer will show up at long last with a fix…. remote / none ??!!

Alan Little

Hoo boy, not an advanced tech genius here.

1. Where to get the original flawed version ?
2. ‘Diff our bootstrap.js files ….. what means this ?

Water park in the Atacama desert. Hrrarramar.

Alan Little

Holy camoley. Thanks Bzzz, but I have no experience with terminal at all. And going to addons / search only brings up the January version of Theme Font, clueless how to find defunct versions…. definitely not showing up on addons

Claus Schottmueller

just installed TB release 60 and recognizes that the Font & Size Changer icon was not there anymore (might had disappeared with a former release – I did not take care). Any case, the addon (I upgrade to 64.3 already) is not working with my TB 60 (no icon and also no menue entry).
Are there already plans to upgrade to a version which is compatible with TB 60
Thx for any information on that issue


Theme Font & Size Changer for ThunderBird 64.1
not showing anywhere in
Thunderbird with Version 60.0

How could I use it?

See screenshot:comment image


Thanks for the reply, but I’ve found a solution that is straightforward to me:

it works without any add on, simply editing and saving a text file: userChrome.css in the chrome folder.

E Nott

This could solve my problem. How do I instal it in Thunderbird 52.9.1 (32 bit ) PLEASE?
Dragging the xpi file to Thunderbird does not work.
Thank you

E Nott

Just what I need.
How do I instal this please as I am not familiar with xpi files.
Dragging it to the Thunderbird page does not work.
Thunderbird 52.9..1 32bit
Hope you can advise, thanks

E Nott

Its simple quick and effective. Don’t know why it was not in the TB program as its essential.
I am new to Thunderbird and the small font made it difficult to read.
Thank you for your prompt reply.
Best regards

Kevin Silver fox

Font Size and Theme Changer is excellent. Thank you.Without it, the stock font size is too small for me to read.

Following auto update 60.2.1 Oct2 2018, Thunderbird has disabled FS&TC.


Installed in TB 60.0b11 (64-bit) on Mac OS X – I used the “Get Addon’s” to do it.

Restarted TB. Nothing changed. How do I find the settings. I’m going blind with the tiny TB fonts.



With the new TB 60 I have the problem that I can set the Zoom, because the up and down arrow near the value are not clickable. Changing the number and pressing Enter doesn’t trigger the Zoom.
There is another way to set the zoom?

TB 60.2.1 (32 bit), TFSC 64.4, W1064bit


PS: i can’T set the zoom (error in my previous post)

The pointer become a cursor over the whole field (up/down arrow included).
Same in Font Size, but if I change the value for font size, then I can Click the “Apply” Button to execute the change.


I’have tested the fixes. The zoom works perfectly now!
Thank you! :good:

Frank Wilson

Thanks a huge bunch …..

I have wasted many hours trying to enhance my TB font size
so that I could leisurely go through the emails of a few days

Yes, I have been using “Theme Font & Size Changer”
for many years

But it is “Fast-Times” nowadays at the proverbial “Mozilla campus”

I am only slightly recovered
from the removal of the “description” field from the FF PW Manager

I’m using Windows and
I am on the TB update channel with version 52.9.1 installed

The “fixed” Addon by Buzz was rejected by FB as being incompatible ….

so I was extremely glad that the above zip download
was successfully installed ….

and now I can easily read the font on my 15-inch laptop screen.

Sure is a configuration smorgasbord for what on the surface appears to a semi-layman
a fairly mundane function (font sizing)

I used to runaway from paying for good s/w,
but after many years and many more hours wasting time
tracking down s/w issues (just like my former working years)

I am now more than willing to donate to developers
who help me use my smart-devices efficiently.

Wish that all developers had a DONATE link
so that I could more appropriately thank them


Shortly after installing this Add-On on MacOS (High Sierra) the folder pane background seemed to become partially transparent (and the message summary pane has faintly darker background for alternate messages).

I would prefer no transparency. Apart from this irritation, the Add-On looks like a good solution for my father’s failing eyesight.
Any hints on how to remove it would be welcome.

(Removing the Add-on did not remove the transparency)


Thanks for the quick feedback. I am also more familiar with Windows than this ‘fruity’ operating system.
I did try completely removing the Add-on but the effect remained… I am asking myself:

1) Did I do something else that caused it? Not as far as I can tell. I don’t think I tried another extension. When trying to get rid of it I temporarily changed some settings in the config editor but they didn’t help and I reversed them.
2) Is there a UserChrome.css file causing this? I can’t find one
3) Was it always like this and I only just noticed? Well I suppose it is possible. When the TB window is in front of a white window it isn’t noticeable. When it is in front of the blue desktop it isn’t noticeable until I foreground the TB window. (The transparency applies some kind of low pass filter on the colour bleeding through so no icons on the Desktop are distinguishable… just a general colouring of the Desktop).

Well – a fix or workaround is available here to reduce the transparency in Desktop windows

So, as you suggest, maybe it is the OS that was doing it.
I consider the issue closed.


Good morning. TFSC not working with latest TBird 68.3.1.



since thunderbird update to 68.4, theme and font size changer isn’t working anymore. Will you update the addon to get working in this new thunderbird version?

E Nott

Any chance of updates soon please for Tbird 68?

Joel Marchand


I am also very interested for a version working with latest Thunderbird (68 and 78). I could eventually pay for that.