Anything containing WebGL and anything containing complex CSS/JS animations comes to mind, also Canvas (even though it rarely used, still lags like a motherfucker), Firefox really suffers in that regard, but they recently promised that they will fix it; and I remind you that because of hardware decoder legal ussues Firefox sucked very hard at 4K and 120 Hz YouTube on Linux for a long time too

There are others, commonly created because Firefox focuses on privacy, and so, for example, all internal website timers can only count by 0.1 seconds because anything less will open you to tracking vunerabilities, often settings sacrifice performance for data safety like this