Firefox does not display specified font (Helvetica)

I updated Firefox to version 10 this evening, only to find that the font Helvetica no longer seems to display where it should on 90% of web sites. It displays as Arial instead. It displays fine on other browsers, and even on limited sites in Firefox, however even setting up a test page with just one line of text and the font set to Helvetica via CSS, the font continues to display as Arial. I've tried the following: - Changing Helvetica to other fonts; they display correctly - Tried Helvetica in different sizes; large or small, it doesn't work Any ideas? Whilst this is a minor issue, as a web designer it's very annoying! Thanks. :)