Sorry, I should know the answer to this, but, it escapes me at the moment. But...
The common suggestion in this situation is to try a new profile. If it is OK with the new profile, it demonstrates that something in the original profile is broken. If it isn't OK, then the problem is in your Mozilla installation.
Of course, a common fix for display issues in Mozilla is to delete the file xul.mfl from your profile directory. This will get rebuilt the next time you start Mozilla.