hello, can you try to replicate this behaviour when you launch firefox in safe mode once? if not, maybe an addon is interfering here...

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

OK, it didn't happen in Safe Mode, so I guess I'm going to have to either reset or figure out which extension is causing the problem. Thank you.

I'm not sure which one it was, because I cleaned up a bunch of unused extensions, but it's not happening anymore.

ok, glad that you could get it sorted this way :-)