Mozilla has “pulled a Microsoft” and put in place a font renderer that makes all text horrible on anything except low-quality monitors. On Windows, you have to “disable hardware acceleration” in the preferences menu (what? you get to choose “slow browser” or “readable fonts”? That sucks).
On OS X … you’re just screwed. Everything is blurry. Some idiot decided to give the world a migraine… The only add-ons I’ve found that let you disable this madness are Windows-only.
I’ll update this post if I find a solution.
UPDATE: using TinkerTool, I’ve been able to globally change some of the OS X fonts, but not the ones that Firefox 5 is using. Still blurry. c.f. Finder, where I can change half the fonts (the time font on the right is Gill Sans, looks great; the font on left is the crappy Apple font that seems unchangeable)