While browsing a website, did you ever got frustrated with the search within that website? Not a problem, let's use the Lord of Search, Google, for all the searching part. This add-on adds functionality to Firefox for Google'ing within a site. Write the term to be searched in the text-box at bottom-right of Firefox and hit enter... so simple. isn't it?
Bonus feature of this add-on is similar site search. So while checking out a website on 'Tennis', hitting this 'similar' button will fetch you all the websites related to Tennis. Again thank Mr. Google :-)
Update [Mar 04, 2010]: This addon has got a "100% Malware Free" award from Softpedia. For details, click here.