Addition to sammingo's explaination Rated 5 out of 5 stars

1. Right click on "Add to Firefox" link and do a "save link". This will create a .xpi file at selected location.
2. Use 7zip or any other unzipping tool and extract the content.
3. Edit the .rdf file as sammingo mentioned.
4. Zip the content back to a zip file and rename it to .xpi (remember to keep the original package structure)
* Alternatively just extract .rdf file, edit it and add back to the .xpi file