Uses an algorithm to determine the most important sentences in a news article and displays them at the top of the news article. Only works for BBC news articles. Isn't a proper Firefox extension either (although you're free to create one).
Everything inside the red box has been selected by the algorithm as the most important sentences, ordered from most important to least important.
Term Frequency * Inverse Document Frequency
I explain all the code and the algorithm in the below blog post. Enjoy