[@Grimhildr] It might be worth while to go through and reinstall your addons, and if the problem still occurs, to reinstall Firefox itself. In my time I've seen some odd bugs and I think the worse one was when all my images somehow pulled the wrong line of code off a website, so smileys were wrong, site backgrounds were made of avatars and all sorts of weird stuff. I think that was a virus cause I had to uninstall Firefox and the residue files to fix it. 

So yeah, start replacing the addons and backup your settings.