Script Summary: Lets you quickly redo your search on different websites (google.ca, amazon.ca, wikipedia.org, youtube.ca, flickr.com or twitter.com) with a right click in any search box. UPDATE: now available as a Firefox addon with way more features!
This script was just my original concept, which I've now recreated in a much more appropriate way: as a browser add-on. Heads up though, until I get 2 more reviews on the mozilla site, it'll require you to log on before installing :(
If you still want to install this script, it also exists in fr, de, co.uk, and com versions.
Click here for .com version
Click here for .co.uk version
Click here for .de version
Click here for .fr version