I require individuals to add links to to my project's wiki page for web sites with OpenSearch plugins that can be "Added" to Firefox's searcbar list of search engines. Web sites with OpenSearch plugins cause Firefox's searchbar drop-down list button to turn blue in color when the browser "auto-discovers" a site with this type of search engine plugin. See the web site list on my project's wiki page for examples of these types of sites.
A web site that supports auto-discovery of a search plugin has a <link> tag in the <head> section of the web page that uses the following format:
<link rel="search" type="application/opensearchdescription+xml" title="searchTitle" href="pluginURL">
Thanks for your contribution to my project!