Lately YouTube on Firefox (Linux) has started to become super laggy. Like keystrokes only having an effect a couple of seconds after the fact. I only noticed this recently and it persists after rebooting. Don't see the same lag in other sites in Firefox and YouTube is fine on Chromium-based browsers on the same machine.
@baldur Not that specific issue, but Youtube has been consistently showing up as the culprit for high CPU use in Firefox for me over the past months, both tab process *and* main process