With the invention of the opensearch specification, it has become super easy to create search plugins for Firefox 2.0+ and IE7. Here's an example:
<?xml version="1.0"?><OpenSearchDescriptionxmlns="http://a9.com/-/spec/opensearch/1.1/"><ShortName></ShortName><Description></Description><Imageheight="16"width="16"type="image/x-icon"></Image><Urltype="text/html"method="get"template=""/></OpenSearchDescription>