When using external CSS (like a CDNed CSS, ext. Webfonts) Firefox throws an error in part of our script:
resp. https://gist.github.com/ydaniv/3033012/#file-mozgetmatchedcssrules-js-L23

There are some hints for this here: http://stackoverflow.com/a/5323868 and http://stackoverflow.com/questions/3211536/accessing-cross-domain-stylesheet-with-cssrules.

The issue, solution and circumvention hack should be well documented so authors don't run into it not knowing what is going on.