After searching around on the internet, I found this post:
The question, how do I change fontconfig to use the same font as my gnome's font settings?
firefox-3.5 takes its font hinting orders from fontconfig, instead of gnome's font settings
SOLVED: The following solved the font problem for me:
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