I had the same problem. Turns out I was missing a couple of fonts. You need Arial regular and also Verdana regular and bold. I just moved these from a Vista fonts folder on another PC to the fonts folder in Windows 7 on my PC and it fixed the problem.
"go to Tools> Options> Content> Font & Color> Advanced And common fonts to fonts such as Tahoma or Times New Roman change you can see other ways in this url:
English Language: http://sinac.ws/
Persian Language: http://sinac.ws/font-problem-in-windows-7-and-firefox/"