Make sure you have the latest version of FireFox (Help / About - check for version 3.6.8)
If not, download it here
http://download.mozilla.org/?product=firefox-3.6.8&os=win&lang=en-USIn Firefox, open Tools / Options
General tab:
Click Manage Add Ons and ensure there are no strange / unneeded add ons installed
Content tab:
Choose Times New Roman
click advanced, choose fonts for Western
check 'Allow pages to choose their own fonts...'
for Default character encoding choose Western (ISO-8859-1)