Not much to say, see test case source.
I see the same thing in WebKit if I enable ligatures (by adding "text-rendering:optimizeLegibility"); basic ligatures are enabled in Gecko by default. So chances are, this is just a broken font file that provides a blank glyph for the "fi" ligature.... ccing Jonathan to confirm.
Yes, Boris is correct. The font contains OpenType rules to automatically replace "fi" (among others) by a ligature, but the ligature glyph (as well as many other glyphs not in the basic Latin-1 set) has been blanked out. This is characteristic of some webfonts from myfonts.com, due to flaws in the tools they're using. Report the problem to your font supplier and ask for a corrected version of the font. (See also bug 697320 comment #7.)