Tab Scope is a cool add-on for Firefox. It allows you to see the contents of a tabbed window without going to that window. When you hover over the tab, it shows a min-view of the window. If you are a heavy tab user, this is really nice since once you have a lot of tabs you can't see the titles in the tab bar. Tab Scope even allows you to do some crazy stuff in the tab like click on links, scroll the page, and navigate forwards and backwards in the history.