So, I gave permission to activate Double-click Image Downloader, but it still doesnt work.

In Firefox, doubleclicking on an image fails to download it.

Additionally, in the Manage Your Add-Ons, Extensions, the Options button fails to appear. (It should be next to the Disable and Remove buttons.) So, it is impossible to tell the add-on to use the hover option.

I notice, another add-on has the same problem, Video Downloader Professional.