When installed, all short urls (eg http://tinyurl.com/123) will be replaced with the urls they actually link to.
Seeing the url allows you to make a better judgement about whether you want to click it. Also some users have services like tinyurl blocked on their networks (for security reasons), this plugin gets around this, allowing them to visit the links.
Uses the api provided by http://www.longurlplease.com/docs to determine determine where the short urls link to.
Download from mozilla addons site at: https://addons.mozilla.org/en-US/firefox/addon/9549