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Are you using scaled resolution? Then it's a known issue that is being investigated and will hopefully be fixed soon: https://bugzilla.mozilla.org/show_bug.cgi?id=1404042

I was using scaled resolution. Switched to default; will report back.

Edit: Been at 100% for 10 minutes. Much, much better. If anyone knows how to set default zoom on Firefox it'd be greatly appreciated.

Try setting layout.css.devPixelsPerPx to e.g. 2 in about:config?

Didn't have any noticeable effect for me.

Well, maybe it already was 2 by default (-1.0 means auto-detect). Maybe try different values.