This code is supposed to be dead since Firefox 3. A quick look at mozilla.org tells me that we just shipped Firefox 17. Need I say more?
Created attachment 683779 [details] [diff] [review] Patch (v1)
This will change our addon story, even though it's hardly an important piece of addon compat. For whoever triages this: addons that provide the "image-sniffing-services" XPCOM category will no longer have any effect in core Gecko code.