Executing this code will allow you to get back to firefox 3.x fonts rendering in firefox 3.5. It also makes Kadu fonts look much smoother.
$ cd /etc/fonts/
$ sudo mv conf.d/10-hinting-slight.conf .
$ sudo ln -s conf.avail/10-hinting-slight.conf conf.d/
$ sudo mv conf.d/10-hinting.conf .
$ sudo ln -s conf.avail/10-hinting.conf conf.d/
$ sudo dpkg-reconfigure fontconfig