have you deleted your firefox profile and restarted it with a new one? i have a fresh install of 12 also and do not have that problem.
i dont like the standard fonts so i go to linuxpackages.net and download cabextract and core_web_fonts, then from a root command line in the directory you downloaded the files to, do the following
Code:
mkdir /usr/X11R6/lib/fonts/TTF
installpkg cabextract-1.2-i486-1kjz.tgz
installpkg core_web_fonts-20050922-noarch-1rsa.tgz
when that is done, copy the font files in the above directory and paste them to /usr/share/fonts/TTF and restart the x-server (ctrl-alt-backspace)
this might not fix your problem but it adds a few more fonts and they look better than the default IMO
hope that helps!