As a security reason, Firefox do not allow an @font-face rule to load fonts hosted on a different domain (even a subdomain).
I don't exactly understand why, I guess it has something to do with preventing crosslinking and copyright violation. I think we should keep the website author handle all this stuff and not required the browser to make assumptions like that.
Anyway, I recently tried to move my CSS file to a subdomain, to reduce pages loading times. Doing so I saw that my fonts did not correctly load on Firefox.
After some digging, I found that I had to manually allow them to be linked from an other domain, server-side. Here is the little snippet I added to my .htaccess
<FilesMatch "\.(ttf|otf|woff)$">
<IfModule mod_headers.c>
Header set Access-Control-Allow-Origin "*"
</IfModule>
</FilesMatch>
