I tried safe mode (all add-ons disabled) and that made no difference. However, I run this same exact setup (minus the KDE Connect and Plasma Integration add-ons) on Windows, even the same theme, but nothing like this whatsoever happened on Windows.
This is weird. I've also just noticed customizing the toolbar no longer works properly either. I tried to move a button around but the button disappears when clicking & dragging and doesn't move from its original location no matter what I do. Wonder if it's related.
I haven't tried a new profile because I was really hoping to avoid that but it seems I have no choice. I'll try it and report back.
P.S. I looked on the Mozillazine Forum before posting here and found nothing like this mentioned.