I've been wondering this as well.
I think it could be in this line, i saw it in the sourcecode of imageshack.us
<link rel="stylesheet" href="img/style-def.css" type="text/css">
<LINK REL="SHORTCUT ICON" HREF="http://www.imageshack.us/favicon.ico">
It seems to be above the rest of the source code (apart from the title), so it seems to make enough sense. But I'm kinda rough on html and everything else to do with web design, so if Kainashi or someone can help me verify this?
Ask on the firefox forums as well, they might be able to help.