Firefox's built-in auto-update doesn't have this problem. It only happens when an external package manager swaps out its files.

See https://news.ycombinator.com/item?id=33201971 for a bit more context.

What does then happen? Do I get some nudge to update?

I for one don't really mind the Autoupdate. I can't stand it that it forces me to restart while I am in some workflow. I'd be fine with "you should update, click here, when ready" which I can click five minutes later when done with the task.

This doesn't work. The only way to disable autoupdates on Firefox is to install a policy file:

https://github.com/mozilla/policy-templates

https://linuxreviews.org/HOWTO_Make_Mozilla_Firefox_Stop_Nag...

Wow, this actually DOES appear to shut down autoupdating. Thanks!

And after some time it will not render pages. Modern SW is such a mess.

You may still update manually.