(1.) Select Tools --> Add-ons and locate ImageHost Grabber's listing.
(2.) Click "Remove" and follow the prompts including restarting your browser.
(3.) After restart, type "about:config: in your address bar.
(4.) Then type "imagegrabber" in the search bar.
(5.) Right click and select "reset" on each imagegrabber entry.
(6.) Restart your browser again.
(7.) To confirm complete removal, follow steps #3-4 again and if you see no entries then IHG is completely gone.