I've been on the borderline of disowning FireFox because of this forced auto-restart behavior. Vendor lock in has been the only thing keeping me on FF; my beloved browser of the past 15 years. Unfortunately they have recently borrowed too much UX from Microsoft.
Here's my final attempt to keep my browsing sessions uninterrupted:
``` sudo apt-mark hold firefox ```
If any Mozilla employees see this, I am begging you to respect the user's choice to postpone updates to when it works for them. You have interrupted my browsing sessions for the nth time and I can't take it anymore.
If you override my system level package settings then it's time for a permanent uninstall.
If anyone else has found a solution to disable (and truly disable) auto-updates in contemporary Firefox, please lmk