Had some CSS that worked in Chrome, but not Firefox or Opera. Firebug reported this:
Turns out, that my media atributes were incorrect,
will not work in FF and Opera, but
However, if this does not fix your problem, or you are not using media selectors, then it will probably be due to an extension to your Firefox browser. I have heard the Foxyproxy and FireQuery cause this sometimes. Best bet is to remove any extensions you’re not using. If you’re still getting it, then it’ll be down to you to decide if you want to use the extension or get rid of the errors.