Why don't you choose a different URL shortener in the addon's settings? I did not cause is.gd to break, I'm just reacting to their service going away (try it yourself: go to is.gd in firefox and notice their website has disappeared).

Also, if you have a favorite URL shortening service that isn't supported, please file an issue on GitHub Issues for this add-on and I'm happy to take a look.

For what it's worth, I personally use bitly (with an API key that I created on their website) and it works great.