Did you try Bootstrap CDN?
Just include <link href="//netdna.bootstrapcdn.com/font-awesome/4.0.3/css/font-awesome.css" rel="stylesheet"> in your <head> section. Font files will be loaded automatically from CDN, too.
Checked this on Firefox and it works perfectly.
@Boris says:
Firefox only allows cross-domain linking of fonts if the server the font is on sends the right CORS headers.
So it's clear that it's CDNs duty to set the right headers, not yours. And them seem to do that correctly, because Firefox doesn't complain.
If you host fonts on your own server, keep in mind that cross-domain rule may still apply, e.g. font files places under domain.com may be unaccessible from www.domain.com, if www.domain.com doesn't send the right headers.
Read this answer for tips about bypassing cross-sub-domain restrictions - this might help in your case.
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