If you're experiencing any sort of major issue with Firefox (pages not loading properly, and other weird behaviour), there's a VERY good chance that some add-on is causing problems.
Try starting Firefox in safe mode and disable any add-ons/themes that may be causing problems. This is done by opening the Terminal and entering:

/Applications/Firefox.app/Contents/MacOS/firefox -safe-mode

Adjust the path as needed if you've installed Firefox in another location.
Firefox's add-on feature is great, but a buggy add-on can also seriously mess things up.