The support for the legacy Sherlock search plug-in format, deprecated since Firefox 24, has been dropped with Firefox 44. The
window.sidebar.addSearchEngine method now just logs a warning in the Web Console when a Sherlock installation is triggered by Web content.
addSearchEngine method still works for OpenSearch plug-ins, but the
window.external.AddSearchProvider method should be used instead as the
window.sidebar object will be completely removed in the future.
addSearchEngine method has been removed with Firefox 59.