The Problem
Ubuntu has a very annoying bug with font-rendering and Firefox: All the fonts in Firefox are blurry, no matter which hinting you choose. I saw this behaviour in 9.10 and 10.04-rc:
The Solution
delete /etc/fonts/conf.d/10-hinting-slight.conf and restart your Firefox, the ugly fonts are propper now!
# rm /etc/fonts/conf.d/10-hinting-slight.conf
That's not really propper...
No, deleting a symlink in /etc is not really propper. As soon as somebody tells me a propper solution to this, I will post it here, too. Normally I don't use Ubuntu, so I personally will not seek for a better solution here. I could file a bugreport, but I you need a launchpad-account for this, which is something I definitively don't want.
Update
There are is an unassigned bugreport about this, with an even dirtier solution.