My first piece of advice for Firefox problems that can't be solved in the usual ways (Checking preferences, updating your extensions & plugins) is to mosey on over to the about:config page. You'll get a big scary warning about voiding your warranty, so dismiss that. Once here, what you'll find is a huge array of settings - fortunately, you can search to narrow things down to what you're looking for. That's exactly what I'd do here - start by searching for font.