If you want to quickly make a bookmarklet get Firefox to use some search engine that nobody else has made one for yet and don't feel like mucking around with Javascript, here you go. This should make searches that work on any engine that uses the HTTP GET method of submitting data. Fill out the form, then click "Make It".
Note: I didn't do much in the way of error checking here. If you submit something stupid, it just isn't going to work.
If it worked, you should be able to highlight any text on a webpage and then execute a search for it by clicking on the button on your menu bar. Alternatively, if you click the button without highlighting something, it will pop up an ask you what you would like to search for.