Extension also include themes, so did you try switching to a default theme, if any other ones are installed? Another thing is Safe Mode disables hardware acceleration, so this is the second thing you can try.

See Troubleshoot extensions, themes and hardware acceleration issues to solve common Firefox problems.

There may be a few more ("hidden") settings that get set back to default in Safe Mode, but most often it’s one of the two reasons above.

I've had the same theme for a long time now.

Nevertheless, did you try? Try to get to Safe Mode "as close as you can" - it isn’t there for no reason. If things run fine in Safe Mode, you should be able to resolve the issue. Often you can trace recent software updates (including add-ons) that may have caused an issue to start occurring.

Disabling hardware acceleration may help too, and if it does, check to see if your graphics drivers were updated recently or if updates are available instead.

Personally, the first thing I would do is try another profile to see what that does in order to rule out anything related to user settings. Hardware acceleration however is then enabled by default, so would need to be disabled in that case.

I've turned off the hardware acceleration and it seems to be working again. Thank you. I wonder why it stopped after being ok for such a long time.