When I want to skip a video in a youtube playlist, skip to the next track or to the previous one in spotify etc, my keyboard media keys don't work. The play/pause button works and so does volume up/down. The overlay appears all the time, but the buttons for forward/ backward are greyed out. Sometimes a song stops when I hold forward for a few seconds. Another guy had this problems 7 years ago, the solution was to start firefox in safe mode. I did that. Didn'T work. I also disabled hardware acceleration, didn'T work.

edit: the media keys work fine in other browsers like edge etc.

Is hardware key support enabled or disabled ?

You can check media.hardwaremediakeys.enabled on the about:config page.

Note that you will likely see a Firefox label next to the controls if you enable this feature and make (volume) adjustments in the player.