If you go to any of the English Moodle Docs pages the Firefox search bar will offer you to install a search plugin named "MoodleDocs (English)". However, if you proceed you will get an error message.
The link tag in the head section of the web page looks like this:
<link rel="search" type="application/opensearchdescription+xml" href="/en/opensearch_desc.php" title="MoodleDocs (English)" />
However, the href attribute points to a non existent PHP page ("opensearch_desc.php") instead of to the XML file of the plugin (see https://developer.mozilla.org/en/Creating_OpenSearch_plugins_for_Firefox#Autodiscovery_of_search_plugins).
This and other plugins can be found at http://mycroft.mozdev.org/search-engines.html?name=moodle for integrating them in the respective web pages.