Starting with Firefox 24, Sherlock-format search engine plug-ins are no longer loaded from local files. Sherlock support and the
window.sidebar.addSearchEngine function, which allows Web pages to install Sherlock plug-ins, will also be removed in the near future, along with the removal of the non-standard
window.sidebar API. Web publishers should provide OpenSearch plug-ins instead.
Update: The Sherlock support has been removed with Firefox 44.
Update 2: The
addSearchEngine method has been removed with Firefox 59.