Have you checked if the same happens with IE? If yes, some of your fonts are corrupted. You can fix this by extracting the fonts from Windows Vista installation disc (if you have it) or use your favorite search engine and do a look up for Microsoft Core Fonts and Microsoft ClearType Fonts Collection. I assume you already know how to install fonts.
If IE doesn't show the same problem, however, you can first try to change the default font in Firefox ( Tools > Options... > Content > Fonts & Colors > Advanced... ) to any other font, restart, change the font back as before, and restart again. If this doesn't work, you'll have to wipe your Firefox profile, and create a fresh one. See here for more information on that.