After upgrading to Fedora 16, I noticed that the font rendering looked awful in Firefox. Even an upgrade to Firefox Aurora made no difference. Purely out of desperation, I tried a method I had used previously to fix a similar issue with Chrome, (http://www.lucidelectricdreams.com/2010/09/ugly-font-rendering-in-chrome-linux.html) and hit the jackpot!.
The original post by Zach Beane can be found here and all credit should go to him.
Save the following to ~/.fonts.conf:
<edit name="autohint" mode="assign">
<edit name="hinting" mode="assign">
<edit mode="assign" name="hintstyle">