Since updating to FireFox 4, I’ve noticed a very irritating fuzzyness in many web fonts. It seems to be a common complaint.
Suggestions floating around on the interweb include fiddling with the ClearType tuner and installing IE9 to get the updated DirectWrite. Neither helped.
What did help – a lot – is turning off hardware accelleration. No doubt at the expense of graphics performance with some current (or maybe future) web content.