Firefox refused to display my Font Awesome icons. After some research I found it was a cross-domain issue created by the fact that the files were being served from a CDN. (Specifically, WP Engine’s CDN.)
The fix for me was to place an .htaccess file inside the font directory containing these lines:
<FilesMatch ".(ttf|otf|eot|woff)$"> <IfModule mod_headers.c> Header set Access-Control-Allow-Origin "*" </IfModule> </FilesMatch>
This cleared it up for me. All now works as it should.
Catalin Rosu nicely explains the situation and the reason behind it here. (Firefox is actually behaving as it should.) If you’re using an nginx for your webserver, Rosu also mentions a possible additional required step.
One other potential alternative option is to embed the fonts as base-64.