If your Firefox interface font somehow become bold after Snow Leopard installed (or its updates, 10.6.2 for example), and you also have bold (or italic) fonts in several sites that is using Helvetica, you need to do this small and easy fix.

Open the Font Book application and make sure you don’t have Firefox active.

Find "lucida" (this is the font in Firefox interface), and delete Lucida Grande (Regular) by right click & click "Reveal in Finder", you will have to enter administrator passwords to delete it. And do not worry, you don’t even have to make a backup, because Mac will recreate this font for system purpose.

Then reopen Firefox, the UI should be back in normal again. Next step is to re-do the above action for Helvetica font or any other font that looks "unusual".

Good luck.