Firefox freezes on Big Sur

Boot into Safe Mode by booting with the Shift key held down and check there to see if the problem persists.  Reboot normally and test again.

NOTE: Safe Mode boot can take up to 10 minutes as it's doing the following; 

• Verifies your startup disk and attempts to repair directory issues, if needed

• Loads only required kernel extensions (prevents 3rd party kernel/extensions from loading)

• Prevents Startup Items and Login Items from opening automatically

• Disables user-installed fonts 

• Deletes font caches, kernel cache, and other system cache files

Downloading a second Firefox seems to have worked. My mac mini wouldn't ask me which to use and substituted the first Firefox even though I removed the old icon from the dock and stuck a new one in. But when I finally tossed the first copy in the bin, everything clicked. 24 hours now and no hangs.

Thanks!

Are you running any anti-virus product? Have you removed FIrefox 84.0.1 and downloaded a replacement? Start with a clean version of Firefox 84.0.1, and the first time that you launch it, press the shift key to prevent it loading its previously saved application state. Let's see if a clean, no-addon Firefox stops the freezing issue.

When was the last time that you booted into Recovery and ran Disk Utility First Aid on your startup drive?

Any add-on that is, or contains 32-bit code, including any 32-bit library, will bork Firefox on Big Sur.

Start looking at the add-ons that you have placed in Firefox. If any are 32-bit, that will cause a problem. I am running Firefox 84.0.1 on an M1 Mac mini with Big Sur 11.1, and it never misbehaves. The only add-on that I have installed is Grammarly.

If you download and run the free GO64 app it will tell you if a 64 bit app has any 32 bit components with it that will not run under Catalina and later. It might tell you if the extension is 32 or not.

Firefox browser works fine on my mac mini OS11.2 so long as I never leave it on the screen when not in use. I just have to remember to "disappear it" by sending it to the margin. I don't think this problem is on Apple, because Safari can be left open 24-7 and doesn't freeze. Left open and unused on screen, Firefox will freeze after a few minutes and must be quit and reopened.

We can't tell either, as only the developer knows how s/he compiled it, and what libraries were used in building it. It was last built in June 2020, so that indicates that it was not built or tested with FIrefox 84.0.1 or even Big Sur.

I have a new apple mini mac (M1-2020). Firefox 84.01 (64 bit) freezes up if open but not in use for a time. Only remedy I've found is quitting and reloading Firefox. Also Firefox gets wiped clean if I restart the mac. I have to start all over with new preferences, password and syncing. On mac restart, Firefox forgets absolutely everything it is supposed to remember.