How to find out which Firefox tab is using 100% CPU without closing any tabs?

Question

Mozilla Firefox

The browser itself shamelessly lags, but it works. You can even type text in forms, all clicks pass, despite hard freezes for 2-3 seconds. Lags continue constantly, even in the background.

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In chrome, this is easily checked, but in Lisa ... Maybe it's possible somehow through devtools at least?

Answer 1

It was an extension console (although it could just as well have been any site) that displayed a unique message every 3 seconds. After more than a day had passed, each new message hung the entire browser completely for about 2-3 seconds.
The button to clear the console in devtools helped.

Answer 2

about:performance