Apture Highlights is a Firefox extension that turns your cursor into a powerful search tool. Highlight any text in your browser window and see search results from Google, YouTube, Flickr, Twitter, and Wikipedia.
Once the app is installed you just need to highlight some text (and not very neatly at that, Apture Highlights will intelligently expand to grab the text you were trying to highlight) and then click on the search box that appears near your highlighted text. You don’t even have to leave the page you’re browsing to view the search results; they’re nested right within the current page.
I use this extension, and it's great! Can also be used as a spell checker, although it wasn't intended as one (ZubaZ this means you!).
Apture Highlights is a free extension and works wherever Firefox does.
Apture Highlights [Mozilla Add-Ons via Firefox Facts]