Doesn't apt also update itself?
When Ubuntu prompts me to restart the system after an update I can dismiss the dialog even for weeks, but at a certain point the cumulative updates start making the system behave erratically and I have to restart. Probably kernel, drivers, libc, other vital stuff get too much disaligned.
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.
Is it very not true? Apt restarts processes and services all the time during upgrades.
I don't think it's true, I had the disable unintended upgrade on Ubuntu because it runs cpu at 100% and make very big noise at night.