window.external.IsSearchProviderInstalled and window.external.AddSearchProvider will both be defined as methods that are no-ops. This was never implemented in Safari. In IE10, these methods are (mostly) no-ops: IsSearchProviderInstalled always returns 2, and AddSearchProvider always returns S_OK. Firefox still implements this, but notes that it may be removed at any time.
Docs: https://msdn.microsoft.com/en-us/library/aa744112(v=vs.85).aspx https://msdn.microsoft.com/en-us/library/aa342526(v=vs.85).aspx https://developer.mozilla.org/en-US/docs/Web/API/Window/sidebar/Adding_search_engines_from_Web_pages