News from the friendly crackpots over at Mozilla:
You are receiving this email because you are listed as a developer of a legacy add-on on addons.mozilla.org (AMO). Mozilla will stop supporting Firefox Extended Support Release (ESR) 52, the final release that is compatible with legacy add-ons, on September 5, 2018.
As no supported versions of Firefox will be compatible with legacy add-ons after this date, we will start the process of disabling legacy add-on versions on AMO. On September 6, 2018, submissions for new legacy add-on versions will be disabled.