Hich CPU usage by GitHub tabs in browser

In the last few weeks I’ve been noticing that browser tabs in GitHub seem to cause high CPU usage (or, as Firefox’s “about:performance” page puts it, high “Energy Impact”). Higher than Youtube, Twitter or Google Docs tabs!
It’s interesting because not that long ago GitHub pages were safe ones to leave in the foreground without causing battery drain. Now they are offenders even in the background.

This happens in MacOS 11.2.3, Firefox 87.0.

I see a similar report for Linux and a Firefox-related browser: GitHub consuming 100% of CPU core, won't relinquish unless entire browser closed

Any idea of what has changed, whether it’ll change back and/or whether it’s fixable with some userscript or whatever?

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