> When Firefox is updated by apt on 20.04, it keeps working until I open a new tab. Then it shows a page asking me to restart the browser and won't do anything else until I comply. This is a choice of Mozilla.

Parts of the program were updated and now the program is in a not tested, half-updated, half-old state.

The "other choice" for Mozilla is pretending nothing happened, attempt to open the tab, probably immediatly segfaulting and losing all your current work in a potentially unrecoverable way. Not really a good choice under most circunstances.

It started doing that only a few months ago. Apparently it worked in the half updated state before then but of course the implementation and the update process could have changed.