hello, do you use a custom theme - can you try to replicate this behaviour when you launch firefox in safe mode once?

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

I don't use any custom themes, and this behaviour does not happen in safe mode, but that doesn't really help does it?

in this case the obvious other candidate would be an incompatible extension - can you disable them one-by-one to find out which one is interfering.

alternatively you could try to reset firefox (this will keep your bookmarks & passwords): Refresh Firefox - reset add-ons and settings

so I did a reset firefox and it didn't help one bit, but it did screw up my layout and configuration - brilliant NOT!