The code for the forums contains this in <head>
Code:
...
<link rel="Shortcut Icon" href="favicon.ico" type="image/x-icon" />
...
<link REL="SHORTCUT ICON" HREF="/images/cKicon.ico">
...
While Chromium based browsers correctly pull (you need an account to see links) (which is a PNG), firefox tries to pull (you need an account to see links) which does not exist.
The second link can either be removed, or the same favicon can be placed at the /images/cKicon.ico path which will make the favicon work in firefox.