How to override website fonts in Firefox?

"You can override any website with your font choices in Firefox.

Open the Tools->Options menu.
Select the ""Content"" tab.
Click the ""Advanced"" button in the ""Fonts & Colors"" section.
Specify the fonts you would like for each of the types for your encoding.
Uncheck the check-box labeled ""Allow pages to select their own fonts...""."

"Check out Stylish, an add on for Firefox:

Stylish allows easy management of user
styles. User styles empower your
browsing experience by letting you fix
ugly sites, customize the look of your
browser or mail client, or just have
fun."