I looked at an application today in both IE and Firefox. Well the fonts look like crap in Firefox, and seem to always look like crud in Firefox. So I finally googled why...turns out the problem is with Windows XP and not Firefox.
Here is the 5 second fix if you can wade through the ads-
Wish I would have googled this a long long time ago...
Note if you change the settings, the setting change is global and will affect more then just Firefox. You might possibly be able to download the power toy here-
Which might give you some additional options, but I haven't played with it.