Theme Font & Size Changer for Thunderbird (#E6F2)

Aaaand it’s finally available as a direct download for Thunderbird (and FF / FF ESR up to 56.x) – use the addon page in your TB/FF or have a look here:

That only took 13 DAYS to approve, I’m impressed. Go Mozilla, you guys are awesome! (I kept one of the keywords for that fuckup, tyvm)

[Original post is here]

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[…] 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 […]


stopped working with TB v60

[…] post wraps up the former #E6/#E6F1 and #E6F2 posts that still contain the files for manual downloading…let’s say for the […]

R. Nathan Brin

I have Thunderbird 64 bit version 63.0.3 How do I change fonts to actual numeric sizes It seems the Thunderbird program designers have once again shamefully forgotten this important function.

R. Nathan Brin

Version 6.3.2 of the 64 bit Thunderbird release seems to have been released in an unfinished state. I am unable to set numerical font sizes for outgoing messages. Some idiot only allowed for sizes like small and large with only a five selection spread. I would like to be able to set point sizes from 1 to 100 as in prior versions. Is a Microsoft programmer involved somehow? There used to be an idiot nicked “savanty claus” clause who thought emails should be plain and barely readable. I hope he didn’t get involved with Thunderbird. In any event, is there a fix to restore Thunderbird to FULL font functionality?


I’ve been using this for several months now. However, something in it broke today, and I’ve been unable to make it work. There is a notice out from Mozilla about a certificate signing……


I’ve got this installed on Firefox 56.0.2 (32-bit) I have a messageand a red screen on the Firefox Extensions page that says “Theme Font & Size Changer for Thunderbird could not be verified for use in Firefox and has been disabled. I was able to get it to work temporarily by chaning my system date back to April 30 and re-starting Firefox. But then I can’t open web sites because of the date difference. Changing the date back to May 4 has Firefox and the extensions working again. According to other sources, the date “only” gets cehced once a day, so I guess this is part of my daily ritual: changing dates back and forth. BleepingComputer has put out this:


Understood, and your suggestions seem reasonable. I had already planned to address my Firefox issues, but I’m a few months late on a coding project that has priority. I just didn’t expect my browser to go all-dramatic on me in the middle of the project LOL. I’m monitoring the fixes that Mozilla are putting out. It seems that the blog over at the TOR Foundation’s site has been active over the issue, too.


Any chance for an update for Thunderbird 68.2.2 ??

Bob H

Try the third post in the link below. It worked for me.

In Windows the profile is in %appdata% -> Thunderbird -> (random string).default


Thanks for your post. The fix posted does not work for me. Very upset that a basic function like this is not part of the basic client. Older people are screwed.

Bob H

Keith –

Sorry to read it didn’t work for you. It worked for me the first time. Only problem I had the font was too big so I had to reduce the number from 20 down to 18. I still might go to 16.

All I can suggest is double checking the directory name and file name. Hopefully you pasted the text into the file to eliminate typos.

I have Windows 10, not a LINUX system that the post was written for.


Hey buzz – nice add-on. Yet, it stopped working with the latest TB update to TB68.3.1. – any idea what I could do? Christian


One workaround for now for scaling:

Go to advanced preferences in Thunderbird and click on Config Editor.

Find “font.size.systemFontScale” and adjust the scale (do so *gradually*! Try maybe 10-15 increase/decrease at a time until you get to where you want.

Changes will show when you close the Mail window and reopen it (or restart Thunderbird).


worked for me – thanks a ton!!!


Great tip! I reset the scale to 125 (the default setting is 100) which makes TBird legible again until the new version of TFSC is available.


Dan from NYC


The “font.size.systemFontScale” parameter works!
Either this setting wasn’t available or didn’t function in earlier Thunderbird versions. But now with version 68.3.1, it does.
Thanks for posting this. It’s just what I needed.


Which begs the question why hasn’t Mozilla made this feature easily available in TBird. Most users won’t want to play around with the config settings.

Dan from NYC

James N

You are the man at the hour. It works. My eyes can’t stand the small fonts for long. Bravo !!


Thank you for posting this!

Kate Camp

Yay! One additional instruction for those of us who really need every single step explained: Double click (on Mac) to open the dialog box where you can increase the scale. I went up to 150 and I can see clearly now. Thank you! :yahoo:

Harry Snodgrass

Extension just got disabled and doesn’t work with the latest update as of 1-14-2020. Any idea if the extension will be updated?



Any chance of an update for 68.4.1?