When someone upgrades TB, and the new TB version requires a new version of an add-on, then that add-on suddenly becomes incompatible and disabled in TB until TB gets around to checking for add-on updates.
People keep asking me "when are you going to make your add-on compatible with TB 68" when it already is and TB just hasn't updated it yet.
This became a much bigger problem starting with TB 68 because so many add-ons require separate XPIs for TB 60 vs. TB 68, and indeed, the recommendation that was made to add-on maintainers was that trying to make XPIs that are compatible with both TB 60 and TB 68 is discouraged.
This is going to be a big problem with the jump from TB 68 to TB 78 as well, given that every add-on is going to have to be converted to a MailExtension.
The fix for this is to have TB check for add-on updates on startup whenever it is upgraded, and if a restart is required after the updates are downloaded, pop up a message telling the user that.