I don’t mess with fonts too much. If I need something custom, like a logo, I’d rather use an SVG image than embedding fonts.
So… are you using an embedded font or are you just using web safe fonts?
If you’re embedding fonts, or loading them from a third-party, then maybe load up the web inspector to see if they’re actually being loaded.
If you’re using Web fonts, then maybe see if Firefox supports it. Although, the whole point of the standard web safe fonts is that the browser is supposed to have it.