@usr11elf Indeed the delay is in the testing and approval process - and unfortunately this takes a certain amount of time. Firefox is given higher priority than Thunderbird because it is a lot more popular than Thunderbird is seeded on the desktop media - and is the default web browser for Ubuntu. With the rise of web-based mail services like gmail etc, desktop email clients have become a lot less popular than they were 10-20 years ago. Thunderbird has not been seeded in the desktop image since before 18.04 LTS as a reflection of this as generally users are not using desktop email clients - most just use the web browser to access their webmail.
As such, testing etc of Firefox updates (and other desktop packages seeded on the desktop image etc) are prioritised over Thunderbird so in general Thunderbird updates will lag behind Firefox updates. Unfortunately in a world where new security issues are found daily across the vast array of software that is distributed in Ubuntu, there is a constant stream of security updates which need to be prepared, tested and released by a finite number of developers. The security and desktop teams have to prioritise which packages to target and so we prefer to take the approach of protecting the greatest number of users as possible by updating the packages that are most used first. Hence why unfortunately Thunderbird updates generally will come later - there are just a lot more users of Firefox (and many other applications) than Thunderbird.
If you are still concerned, the immediate steps you could take would be to either:
- Consume the pre-release updates from the Mozilla Security PPA - but note these have not gone through the full validation so may have stability or other issues
- Switch to the Thunderbird snap - this is generally more up-to-date as it can use the same set of dependencies across all releases and so doesn’t get held up when Mozilla decide to depend on newer technologies/packages than are available on older Ubuntu releases like 18.04
- Switch to using the binaries provided by Mozilla