Search Again од Jason Skomorowski

Firefox makes it easy to run searches on different sites--you can even assign shortcuts to them in Preferences -> Search. For example, you could set 'y' as the keyword for "YouTube" and type "y my search query" in the address bar to run that search on YouTube instead of your default search engine.

Where this add-on comes into play is if you want to run the same search again on a different engine. If you have your keyword for "Google, limited to the past three months" set to 'g3m' and want to run the query you typed for YouTube on that instead you cannot just change the 'y' to 'g3m' because your query has already been replaced with the address of the search results. With this installed, you can type "sa g3m" and it will attempt to read your query from the URL and run it again on the search engine associated with 'g3m'.