As the title states, if you click on a link using middle-mouse click, Firefox will open it in a new tab. This is awesome for Google searches as you can open the first 10 results in tabs, and proceed to the next 10. Once you have 20 open, you can Control + W, and start reading your results to your search.
Also works good for Blog Aggregators, like Fullasagoog.com for instance. You can pick entries you wish to read, and then when your done picking, go through each tab.
Quicker than Right Click + Open in New Tab.