Similarly to what was discussed earlier this year, we have decided to update Thunderbird in all supported Ubuntu releases from 78.x to 91.x. This is a major version update.
Thunderbird 78 is no longer supported by upstream, the last release in the series (78.14.0) having been released over 2 months ago. We don’t expect it to be practical to backport security fixes to LTSes indefinitely. It has therefore become necessary to move to Thunderbird 91.x.
This major new version does bring a number of improvements, UI changes and bug fixes, but we don’t expect any major disruption, unlike in the previous transition from 68.x to 78.x.
One known functional regression is the removal of Movemail support.
Users of Ubuntu 18.04, 20.04 or 21.04 not wishing to update beyond Thunderbird 78.x may instruct apt to hold or pin the package, but this is not recommended as the package will no longer receive security updates.
This decision was discussed with the SRU team and it was agreed to use the normal security exception process, not the SRU process. It is tracked in bug #1949605 .