There was a weird thing which occured for somepeople so worth seeing if the following fixes your issue.

See if this fixes the issue: Settings > General if not selected then select checkbox to enable: 'When thunderbird launches, show start page in message area'

then for Location: remove all the text in this area so it's completely blank. Then uncheck to disable: 'When thunderbird launches, show start page in message area' Restart Thunderbird and report on whether this fixed the issue.

No, it did not solve the issue. I'm guessing that if I had allowed 115 to install I would have that screen and a checkbox somewhere that would disable it in future.

Sounds like you already have problems, so why not bite the bullet and install 115. I really do not see 115 as any worse than previous versions. It is ugly and has all these new ideas that do not agree with how I do things, but issues abound with every release and generally once the initial few months of fixes come out things only get worse the longer you delay, not better as is the common thinking. That is one of the reasons the developers do not wenable update notifications until they get to the .1 iteration about 4 weeks after launch, to let the unforeseen appear on new installedversions before the majority of users are affected by updates. The development teams are already very risk averse. What I do see is a lot of very squeaky wheels that represent very few actual users that make a load of noise about their pet peeve. But one instance of folks that made things worse by not upgrading were those that skipped V68 entirely and then could not get passwords to save in later versions.

Keeping things up to date is something that actually works for you when seeking support as those who do volunteer to do support tasks have fresh memories and current knowledge. Beyond that we have no more special access than you do so as time passed from when the issue was"new" our memory like everyone's generally fades over time.

BTW, unless you ensure that your "apps" are not still running in task manager as applications or tasks, the absence of a user interface is not evidence the application has closed. I have closed Firefox on occasion and still had many tasks still in task manager. It is far simpler is to simply turn the device off and let it's capacitors discharge. Even now a truly large number of issues are fixed by a simple power off cycle, and boot times with SSD drives are actually almost as fast as a resume from sleep. Hence, I think, Microsoft forcing a reboot every month, and making no attempt to remove the need.

However to the question at hand. This what is new appears to come from this implementation bug https://bugzilla.mozilla.org/show_bug.cgi?id=1552725 Comment nine has instructions for fooling the software that an update is available, and mentions the app.update.postupdate hidden preference being set to true. So perhaps check in the config editor just what value that preference has. Toggling it might be solution you seek. Or it might not. I am not a developer.

I've been checking the difference between the old 102.15.1 version and the beta version 125.0b5.

In Config Editor I located this:

mailnews.start_page_override.mstone If you are using version eg: 102.15.1 then that is number you should see in the value

Do you see a different number from the version you are currently using ?

@Matt, there does not appear to be a value set: app.update.postupdate BooleanNumberString

Setting value to True and restarting did nothing. Toggling to false and restarting did nothing.

@Toad-Hall, No, this is displayed: mailnews.start_page_override.mstone 102.15.1

So, a very persistent annoyance. Thanks for trying to help.