Another idea is to press F12 to open the dev tools, go to the network tab, and refresh the page. If something is not loading it will be highlighted in red and it will tell you the reason.
It might be some addon messing things up, you can launch firefox with no addons with firefox --safe-mode
.
Does it happen in a "private" window?
It sounds exactly like that. Perhaps you ran firefox as root at some point?
I'd say:
~/.mozilla
:/.mozilla
:If you get permissions errors, run them with sudo.