Why does Firefox automatically update when I have selected for that NOT to happen?

Hello

File a bug report or feature request for Mozilla products https://support.mozilla.org/en-US/kb/file-bug-report-or-feature-request-mozilla If you wish, you can send your comment, file a bug report

Just for information

Firefox release notes https://www.firefox.com/en-US/releases Since the release of Firefox 63 https://www.firefox.com/en-US/firefox/63.0/releasenotes

The option to Never check for updates was removed from about:preferences. You can use the DisableAppUpdate enterprise policy as a substitute.

I choose my favorable moment to perform the updates. I use, DisableAppUpdate https://connect.mozilla.org/t5/discussions/upgrade-notification-spam/m-p/64080/highlight/true#M22529

If you want to study Enterprise Policy Generator https://addons.mozilla.org/firefox/addon/enterprise-policy-generator

For information https://connect.mozilla.org/t5/discussions/enterprise-policy-generator-an-open-source-browser-based/td-p/127768

Among other issues https://support.mozilla.org/en-US/questions/1588944

Is this specifically about the "Restart to keep using Firefox" about:restartrequired tab/process dead end?

I assume this is not an enterprise install where the updates would be distributed by your company, as you'd be in touch with your IT instead in such case.

Quick check: You might be running multiple profiles at once in different windows. Make sure ALL of the profiles have the same setting to not update, so that any one of them is not starting the update process on its own (which then renders the other profile unusable until restarted). Even then just launching a separate profile after it pulled the update before while running earlier might trigger the "oh cool now I'm restarting" update trigger which is basically "Firefox updates when launching new profile, even when other profile is open" bugzilla.mozilla.org/1758669 that eventually got included under "Second instance of Firefox will apply update downloaded by still running first" bugzilla.mozilla.org/1480452 in general.

If none of the above sounds like your situation, e.g. you're positively definitely not running separate profiles/instances, there's something off that needs looking into.