Strange. Obviously a problem with the actual program, or the interaction of this particular program with your computer. A couple of suggestions maybe, to cover both eventualities.....
1. Firefox > Tools > Options > Content > Fonts and Colors > Advanced...
i) Check to see if 'Fonts for Western' (top)
ii) Check to see if 'Default Character Encoding: (Western ISO-8859-1)' (at bottom).
iii) If 'yes' to above, try experimenting with a few different fonts and colours. Maybe likewise with the 'Allow pages to choose their own fonts.....' option.
2. Uninstall/reinstall
3. Try downloading FirefoxPortable version -
http://portableapps.com/apps/internet/firefox_portable - and see how that interacts with your computer.
NB: Although designed for flash drives etc (where you could trial it from, provided the main edition of Firefox on your PC is not running), it also works perfectly well installed on the computer (again, provided the other one is off). Being portable, it's fully self contained within its folder, so wont affect or write any settings to your machine (or interfere with the main edition).
Good luck.