In versions of Firefox prior to Firefox 3, extensions could add decoders for new image types. However, such decoders relied on servers sending correct MIME types; images sent with incorrect MIME types would not be correctly displayed.
In Firefox 3, a new XPCOM category was added in bug 391667 to allow "third-party" XPCOM components to identify images based on their content. If Mozilla's built-in image code cannot identify an image, then any XPCOM components implementing the nsIContentSniffer interface may register under the "image-sniffing-services" category, and will be queried to determine the MIME type of the image being downloaded.
Note: This category was removed. See Bug 813787 - Remove support for image-sniffing-services.