If you tend to use a few different search engines, check out Drag & DropZones, a nifty Firefox add-on that makes searching for snippets of text in a variety of search engines a snap. It works by dividing the browser window up into a series of “dropzones;” one for each search engine that you use. Grab some text and start to drag it with your mouse; the dropzones will appear. Drop the text a zone and the appropriate search will be performed in a new tab.
Out of the box, Drag & DropZones is automatically set up with your current set of search engines, but you can customize it by repositioning the dropzones, changing the grid size and coloring the zones, as well as altering its behavior, like whether Drag & DropZones activates when dragging images and links.
Drag & DropZones can be downloaded from the Mozilla Firefox Add-Ons Gallery. It works with Firefox 2 and later (including Firefox 4).