Free, simple Firefox add-on InstantFox powers up the Firefox address bar so that you can search many different sites, such as Google, Twitter, Wikipedia, Amazon, YouTube, and more, without having to visit the sites themselves--or even use Firefox's search box. Instead, put a letter in front of a search, type it into the address bar, and off you go.
Simple one-letter codes for popular sites are already programmed into InstantFox. Let's say, for example, you want to search Amazon for the book The Tiger's Wife. Into the Firefox address bar type "a The Tiger's Wife" (without the quotation marks), and it'll search Amazon for that book.
You can search other sites in a similar way. Append g for Google, b for Bing, yh for Yahoo, y for YouTube, t for Twitter, e for eBay, w for Wikipedia, f for weather, i for Google Images, and so on, and you'll do the search. Also useful is that it displays the search results as you type, changing the results with each new letter you type.
The add-on is useful, but it could use some improvements. You can't add your own favorite sites or search providers to the list, for example. Still, this is a very useful freebie, and well worth the download for anyone who often searches using multiple Web sites.