Was great when it worked. Nowadays, I get an error each time I try to add a new search engine: "Firefox could not download the search plugin from:
https://file.io/XXXXXX". I have switched to the "Search Engines Helper" which also uses file.io, but works fine on my system.
UPDATE: in response to dev's question: I can visit that file.io page with no problem. The Search Engines Helper addon also uses file.io as yours does, but it works for me.
UPDATE 2: Note that I can actually manually access the file.io URL that the "download error" dalog gives me by entering it in the URL bar -- I see the source of the XML file. Hope that helps!
UPDATE 3: After updating to new version (1.8), all is working properly again, so I have revised my lowered rating to reflect my true opinion of this very useful add-on.