User Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.15; rv:82.0) Gecko/20100101 Firefox/82.0

Steps to reproduce:

I've notice sluggish performance on JS-heavy sites compared to Safari and Chrome. One site in particular is dramatically worse: https://projects.fivethirtyeight.com/2020-election-forecast/

Actual results:

Mousing around the simulation graphic (the one with all the dots) takes the better parts of a second to show the pop-up for each dot. According to the profiler, the long pauses are in "Paint" calls.

Expected results:

Both Chrome and Safari show these pop-ups almost instantly.

My apologies... I discovered a rogue tab that was responsible for these slowdowns. With that closed the performance is OK.