Using a custom short URL service
By default, Copy ShortURL falls back to the URL shortening service is.gd. As I am not affiliated with is.gd however, I wanted to make it easy for people to use whatever service they like.
You may specify a different service by clicking "preferences" on Copy ShortURL in the Firefox add-ons pane.
There, you can choose from default services or enter your own custom URL.
When creating a new short URL, the add-on will request (GET) the URL you
specify here, while replacing the placeholder
%URL% with the long URL that
it is trying to shorten. As a reply from the shortening service, it expects
a one-line, plain text document containing the shortened URL.
http://tinyurl.com/api-create.php?url=%URL%(3rd party docs)
Popular service bit.ly has a slighty more elaborate API. Create an OAuth access token, then enter the following custom service URL into Copy ShortURL's settings:
<accesstoken> in the URL with the token you created.
Full bitly API docs exist.