If you’ve customized TB with plugins you care about, DO NOT UPDATE to 68 until you verify that every plugin you use is compatible. TB will NOT check for you and once you launch 68, the plugins that have been updated to 68 compatibility will not work with 60.x, which means you better have a backup of your .thunderbird profile folder or you’re going to be filled with seething rage and you’ll have to undo the update. This misery is the consequence of Mozilla having failed to fully uphold their obligation to the user and developer communities that rely on and have enhanced the tools they control.
To avoid this problem now and in the future, you have to disable automatic updates. In Thunderbird: Edit->Preferences->Advanced->General-[Config Editor…]->app.update.auto=False, app.update.enabled=False.
On Linux, you should also disable OS Updates using Synaptic: select installed thunderbird 60.x and then from the menu bar Package->Lock Version.
If you’ve been surprise updated to the catastrophically incompatible developer vanity project and massive middle finger to the plugin developer community which is 68 (and 60 to a lesser extent), then you have to revert. This sucks as 60.x isn’t in the repos.
First, do not run 68. Ever. Don’t. It will cause absolute chaos with your plugins. First it showed most incompatible, then updated some, then showed others compatible, but had deleted the .xpi files so they weren’t in the .thunderbird folder any more, despite being listed and shown incorrectly as compatible. This broke some things I could live without like Extra Format Buttons, but others I really needed like Dorando Keyconfig and Sieve. Mozilla’s attitude appears to be “if you’re using software differently than we think you should, you’re doing it wrong.”
The first step before breaking things even more is to backup your .thunderbird directory. You can find the location from Help->Troubleshooting Information->Application Basics->Profile Directory. Just click [Open Directory]. Make a backup copy of this directory before doing anything else if you don’t already have one, in linux a command might be:
tar -jcvf thunderbird_profile_backup.tar.bz2 .thunderbird
If you’re running Windows, old installers of TB are available here.
In Linux, using a terminal, see what versions are available in your distro:
apt-cache show thunderbird
I see only 1:68.2.1+build1-0ubuntu0.18.04.1 and 1:52.7.0+build1-0ubuntu1. Oh well, neither is what I want. While in the terminal uninstall Thunderbird 68
sudo apt-get remove thunderbird
As my distro, Mint 19.2, only has 68.x and 52.x in the apt cache, I searched to find a .deb file of a recent version. I couldn’t find the last plugin compatible version, 60.9.0 as an easy to install .deb (though it is available for manual install from Ubuntu) so I am running 60.8.0, which works. One could download the executable file of 60.9.1 .and put it somewhere (/opt, say) and then update start scripts to execute that location.
I found the .deb file of 60.8.0 at this helpful historical repository of Mozilla installers. Generally the GUI will auto-install on execution of the download. But don’t launch it until you restore your pre-68 .thunderbird profile directory or it will autocreate profile files that are a huge annoyance. If you don’t have a pre-68 profile, you will probably have to hunt down pre-68 compatible versions of all of your plugins, though I didn’t note any catastrophic profile incompatibilities (YMMV).
Good luck. Mozilla just stole a day of your life.