It's working for me. I just started using it. The Fearch icon automatically appeared on the FF toolbar after installing the addon. However, the results are rather meager and not very exciting or rewarding, perhaps because it can only match on filenames rather than PDF titles and such. I've tried a bunch of different searches, mostly for books.

I think it's very strange that IMAGES are not included as one of the media checkbox items, but I guess you can do the "a.jpg" thing, where "a" is a specific filename, but that is nowhere as good as doing a Google search for images based on an actual search term, rather than a filename.