Editing the install.rdf file to increase the version number does help, but even then, this add-on works only minimally. Users must right-click on the icon, select the capture option they want (complete page/visible page/selected area/entire window), and then click "Copy to clipboard." After that, users must paste the image to an image processor. Only then can the image be saved to disk.
Way too cumbersome and much work. This add-on has not been updated in over a year and has clearly outlived its usefulness in light of recent changes to Firefox. Time to move on to other screen capture add-ons like Screengrab (fix version) and/or FireShot. Abduction! used to be viable candidate, but it, too, now behaves badly with FF18.