This is an already built in function in Firefox Rated 4 out of 5 stars

We can do this without any extension by just going to, searching the site name in its database, and adding the site we want in the results. And their database contain atleast a thousand popular search engines. To the beginners this extension will be very useful. I hope we can add those search engines which are not in their database with this extension. Good work. 6/10

This review is for a previous version of the add-on (1.7).