Puntuado con 3 de 5 estrellas
I think Google removed the Firefox capability.
According to their statement: Google URL Shortener is currently available for Google products and not for broader consumer use.
I opened chrome and installed the extension there and it works fine.
Direct link to Goo.gl in Chrome: http://goo.gl/PiyX
Sucks, because while I use Chrome, Firefox is my main Browser.
Come on Google.