Weird Text appearance after waking up from sleep mode

My iMac 2013 runs on Monterey and somehow after being in a long sleep mode (over night), text in open firefox looks like in the picture above. Mind you not all text... this is just the most extrem website I had open. Weirdly, I had another private window open in the background which wasn't affected. After restarting firefox, all seems to work fine.
I tried to disable feature unlock, no luck here. Anyone got an idea how to fix this? Apart from it, all works fine.