Actually xKiv, I looked at your user agent string and it says you're using Firefox 29. That's actually several versions behind and might be what is causing your issue, I know at one point Font-Awesome and Firefox didn't play well together but in the current version of the browser (31) it works fine.
I know I have had *different* problems with custom fonts on windows firefox (iirc required manual installation of the font) and linux firefox (no idea what I did with that), once.
As for version, I won't be updating firefox until I update the entire system (I am on mint 14, which is no longer supported and therefore doesn't get updates to anything anymore). And I don't want to do that until I manage a complete backup of all my data. And I don't want to do that until I upgrade my network from 100Mb to gigabit ethernet (requires configuring a new modem, and hopefully not much else, but it's still a drag). At my current willingness to do stuff like that (and considering that I already set up a NAS here and configured access from two computers), I *might* get to it somewhen this year.
ETA: actually, it looks good on a clean profile. I will have to investigate a little more. Clearing cache didn't help.
ETA2: restarting in safe mode helped.
ETA3: oh DUH. That's the thing I forgot about "linux firefox". NoScript doesn't just block javascript, but also fonts. I have to explicitly allow them.