The deprecated, non-standard
window.sidebar object will be removed in the near future. The
addPanel method has been removed with Firefox 23, and
addSearchEngine method has dropped the Sherlock support with Firefox 44, so this object is no longer useful. However, apparently it’s still used by some sites for browser detection. Web developers should stop using it.
addSearchEngine method has been removed with Firefox 59.
window.sidebar is no longer available on the Nightly channel as of Firefox 65.