Let's try a few things here.

First, try reading Websites look wrong or appear differently than they should. specifically look at the clearing cache section. Does that help?

If not, Try to start Firefox in Safe mode. This mode disables all extensions, user customizations, hardware acceleration, etc. temporarily for diagnostic and troubleshooting. If Firefox works fine in Safe Mode, then this is likely an add-on issue. Restart Firefox in normal mode, then type "About:addons" (without the quotes) in the address bar. Disable your extensions one by one, restarting Firefox between each one, until you find the trouble maker.

Also make sure you have all Mac OS updates as well :)

Hi thanks for the help.. Unfortunately none of this worked. It's a brand new mac. Before when I had a pc everything worked normal.