@jules:
On my webside
no admin has such a problem using firefox. Only admins can see icons from this font here.
Your problem may be self made.
Hints:
Look for your css files! Test it locally on your dev system, e.g. with XAMPP on windows!
If you see them within the panel, it is your css problem… . If not, it may be a problem with your server…
Good luck!