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However, you may not know that a very wide selection of search plugins are available (click "Add Engines..." from the search field drop-down menu). Once installed, a different search is only a mouse-click away.
Alternatively, it's possible to define keyword searches from the address bar. For example, you could search google by typing "gg webmaster world", and altavista by typing "al webmaster world".
Neither is quite the solution you're asking for, but those two techniques give you lots of search engines whether you prefer the mouse or keyboard, and without sacrificing screen space.
Finally, you could find or make a web-page that does searches, and save it as a bookmark to open in the side-bar. With a little scripting, you could even add it as a proper menu item.
This article at Netscape Devedge [devedge.netscape.com] explains how to do it in Mozilla -- it works exactly the same way in Firefox.
I understand that the latest nightly builds also have a new keyword-related feature to automate your most common searches, but I haven't tried it out yet.