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Do you keep Gmail open? It's poorly optimized for non-Chrome browsers and tends to peg the CPU at seemly random times.

I didnt know this, but im not surprised.

Honestly, at this point, I don't even use Gmail web: I just use the macOS built-in Mail app (Thunderbird unfortunately doesn't have a friendly UI).

If more people switch to Firefox, this might be an additional effect: a rise in the usage of desktop mail clients.

You can always quit Firefox process and reopen it again, unlike Chrome, Firefox doesn't auto load all tabs again, so this will kill any scripts running or monitoring in rest of the tabs. This almost always solves my issues.

If you are on an OSX machine, it may be this bug: https://bugzilla.mozilla.org/show_bug.cgi?id=1404042

If you're on linux you could try moving out your ~/.fonts to see if it's a fontconfig problem

https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=909818

(I've seen variants of this problem on Ubuntu 18.04 LTS.)

about:performance is not very detailed, but it could help pick out a misbehaving tab.