I had the same issue, but only had partial success.
The Font Awesome icons appear in the toolbar editor:
Yet don't display in the editor itself--I only get blank buttons in both the default XF theme (shown, with color changes, hovering over one of the two new buttons, underlined with yellow) or the third-party theme I use. Other Font Awesome icons appear throughout the site properly.
I'm using fa-superscript and fa-subscript, which displays them in the button editor, but not the editor itself.
But I found the problem:
Just figured it out, though--the information above was incorrect. If I change this to:
...without the fa- prefix, it then works in the editor:
It also appears in the toolbar editor properly. Which is odd, since I would think the incorrect code would prevent the icon from appearing in both places. But regardless, this entry only needs the name of the icon without the fa- prefix.
(Side note--I wish that there were help tooltips throughout the admin side, as I had to find this thread to figure out what to enter in this field.)