Did you try to close Firefox normally? Did the browser respond to anything?

Start Firefox in Safe Mode {web link}

A small dialog should appear. Click Start In Safe Mode (not Refresh). Is the problem still there?

When this happens FF cannot be closed normally, it's unresponsive. The problem is random, so restarting in safe mode will likely not accomplish anything?

Are you possibly using Private Browsing mode because this keeps all data in memory and can thus add more memory strain?

opus817 said

The problem is random, so . . . .

About how long before the problem shows up? What were you doing at the time? surfing, watching a clip, walked away, reading an article . . . .

It can happen while I'm not doing anything with FF.

I checked your System Details. You have several Legacy add-ons that are not compatible with Quantum Firefox. Those should be removed.

Just In Case: You may have ad/mal-ware. Further information can be found in this article; https://support.mozilla.org/en-US/kb/troubleshoot-firefox-issues-caused-malware?cache=no

Run most or all of the listed malware scanners. Each works differently. If one program misses something, another may pick it up.

Thanks Fred. Can you point out which add-ons should be removed? There are only a half dozen or so that are enabled.

Type about:addons<enter> in the address bar to open your Add-ons Manager. Hot key; <Control> (Mac=<Command>) <Shift> A)

In the Add-ons Manager, on the left, select Extensions. Anything with the label Legacy should be removed or updated.

Thanks Fred. This is a bit confusing. The list of Legacy Extensions says they have been deactivated, but at least one of them has not (Tab Counter). Otherwise, pretty much all of them are not enabled anyway - could they still have an effect on performance even though they're disabled?

Only their programmers know for sure. A few Legacy may still be able to work, depending on what they do and how they do it.

Tab Counter 0.4.1 is a WebExtension {min:52.0, max:*} Tab Counter https://addons.mozilla.org/firefox/addon/tab-counter-webext/

Here's what CPU and memory looked like a few seconds after opening the same set of tabs in FF and Pale Moon (actually, FF was using 80% CPU 1-2 seconds before this screenshot)

Did you try to expand Firefox to get more detail about the 12 Firefox processes that are active (likely eight content processes and four others like compositor)?

I may be missing something, but expanding FF doesn't appear to provide more info - just lists FF 12 times, with no description of the processes involved.