When requiring a font from an external CSS file (assets domain, CDN, etc), you may see that Firefox refuses to display the font. This is because Firefox, by default, disables cross-domain fonts. To solve this you have to add a header to the response with the font file. The header is Access-Control-Allow-Origin: *

In a typical nginx setup, this is achieved with this snippet:

nginx add header

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location ~* \.(eot|ttf|woff)$ {
  add_header Access-Control-Allow-Origin *;
}   

Once you have that in place, the fonts will be loaded correctly from Firefox.