The LongURL Mobile Expander uses the LongURL.org web services to let you know where shortened links really go. Just install this add-on and whenever you come across a shortened URL, just place your mouse over it, it will reveal the real URL as a popup.
There are several sources for shortening the long URL (for example TinyURL). However in some situations, it may be necessary to know the real URL behind the shortened URL before clicking on it. The Firefox extension LongURL Mobile Expander lets you expand shortened URLs on any website, no matter where you go.