I wrote and/or help to maintain ten different extensions for Mozilla Thunderbird: Enhanced Priority Display, Folder Pane View Switcher, IMAP Received Date, Keyconfig, Remote Content By Folder, Reply to Multiple Messages, Send Later, Show All Body Parts, ToggleReplied, and Undigestify. Of these, Send Later is by far the most popular; with an install base of over 80,000 users, it is the eleventh most popular Thunderbird add-on.
Thunderbird and Firefox share a lot of code. In the past, the code they’ve shared has included the framework for how extensions work. However, Firefox has now obsoleted that extension framework in favor of the cross-browser “WebExtensions” format. This has caused a huge amount of turbulence as the maintainers of Thunderbird have tried to figure out how to continue to support the more than 1,000 old-style extensions which Thunderbird users have come to rely on.
Thunderbird 60, the current stable version which you get when you go to www.thunderbird.net and the version included with most current Linux distributions, is mostly unaffected by the turbulence, but the newer alpha and beta versions of Thunderbird are a hot mess.
The Thunderbird maintainers have attempted to capture documentation of how to port extensions to newer Thunderbird versions at https://wiki.mozilla.org/Thunderbird/Add-ons_Guide_63, but that page leaves a lot of questions unanswered, and even if it answered all of those questions, as far as I can tell things are still so much in flux that making the extensions I maintain work with anything newer than Thunderbird 60 probably isn’t possible right now.
The Thunderbird team really does need to figure out how to make it possible for us extension maintainers to port our extensions to newer Thunderbird versions without a huge amount of effort. Many Thunderbird users rely on extensions, and they are going to keep using Thunderbird 60 until the extensions they rely on are supported in newer versions.
Supposedly the TB team has money to spend and is hiring developers to do the necessary work (see https://blog.mozilla.org/thunderbird/2019/01/thunderbird-in-2019/), but it’s really not clear how long that is going to take. I hope not that long.