Icon fonts are not showing in Firefox. How to fix this?

It looks like it is about the cross-domain request issue.

For the fix, you might need to add some code into the ".htaccess" of your site to allow that request. See here for more info: http://davidwalsh.name/cdn-fonts

Or, if this issue happens on secondary language when you are using WPML plugin, please go to "WPML > Languages > Language URL format" and change the format to "Different languages in directories" instead. It should help.