Related question here.
Boris' answer to the above question seems to make sense, but I installed the Font Awesome files on my server and the problem remains:
I looked into the library and found that the font files are included in the install, so the argument about cross-server access to fonts doesn't seem valid. I don't mind using the BootstrapCDN, but then Boris' suggestion seems to apply, and I don't know how to send the right CORS headers. (I tried it, but it doesn't work either.) Any idea how I can fix this, either using the "Bootstrap CDN" or the "default CSS"? (see also these instructions.)
PS: IE10 shows the glyph correctly.