I can see the issue in Firefox.
First, try removing vault/resources/fonts/font-awesome.woff from your server's cloudflare cache.
Make sure your server is serving .woff files with mime-type: application/font-woff
Cloudflare is reporting a HIT for the file and sending the mime-type as text/plain. I don't know if that's standard for all cache hits, or if it's actually saying .woff files should be interpreted as plain-text, rather than as fonts.
For a Firefox browser with an empty cache, Cloudflare is returning empty Response data for the font in question, rather than the actual content of the file.
I realize now that Chrome was working okay for me because I have Font-Awesome installed in my OS so I can use it in Word and PhotoShop, and it must be using the system-installed version. I'm a little confused why Firefox didn't try to do the same thing.