I had the exact same infuriating experience the first half hour of using my OLED panel but it turns out it was simply because Firefox doesn’t support hdr. You have to use edge or chrome for hdr content online. So now I use edge purely for YouTube and Firefox for everything else.
Off the top of my head: right click the task and hit end process. That has literally never failed me. Back in windows XP it might sometimes not actually kill the process but then there was always the “kill process tree” button to fall back on.