Hi Michael, add-ons are not allowed to inject scripts into certain Mozilla sites because those sites have special access to the browser. For example, the Add-ons site can enumerate your add-ons, and enable or remove them. Allowing add-ons to run would let the add-on manipulate these features, too.

Thank you very much for answering. When I thought about what URL is was trying to shorten, it was one from possible add-on extensions I wanted to email to a friend -- so from your answer I can see why I was blocked.

Michael