Try resetting your Firefox profile. Sometimes a weird setting can break browsers in spectacular ways.

This was a big one for me, for the longest time I could not figure out why I couldn’t get YouTube to play videos over 1080p for me in Firefox on my PC, it ended up being some weird setting that I changed in about:config (I sadly cannot recall which one) a long time ago - but I’d always copied my Firefox profile with me so that bad setting stuck around.