After upgrading to 9.10, the fonts in Firefox were too big and blurry. I fixed it as follows:

cd /etc/fonts/conf.d
sudo mv 10-hinting-slight.conf 10-hinting-slight.conf.original
sudo ln -s /etc/fonts/conf.available/10-hinting-medium.conf /etc/fonts/conf.d/.
sudo ln -s /etc/fonts/conf.available/10-sub-pixel-rgb.conf /etc/fonts/conf.d/.
sudo dpkg-reconfigure fontconfig


Restart Firefox.

Thanks to Carlo Hamalainen for providing the fix.