Check your computer's font settings. Github's font stack looks like this:

Helvetica, arial, freesans, clean, sans-serif

This means that if you have the first one it comes across, it will use it. If you don't have it, it moves on to the next one and so on down the list, until it gets to one you have, or the default system sans-serif font.

If your system font is something less horrid, then check Firefox's default font. In Preferences, find "Fonts & Colors" and check the value of the font and change it if needed. For more info, check out Firefox's help page.