During the initial research for my project, I discovered that Firefox 2.0 detects search engine plugins that may be manually added by the user to Firefox's search engine drop-down list. The user may then decide to select a particular search engine as the default for the browser's built-in search box. When Firefox detects a search plugin that has not yet been "Added" to the browser's list of search engines, the search engine icon and the drop-down list arrow of the browser's search box turn blue in colour.
<head> section of the web page:
<link rel="search" type="application/opensearchdescription+xml" title="searchTitle" href="pluginURL">