There are two seaprate lists of search engines, whose uses intersect
in this file. The search service (nsIBrowserSearchService and
nsSearchService.js) maintains a list of Engine objects which is used to
populate the searchbox list of available engines and to perform queries.
That list is accessed here via this.SearchService, and it's that sort of
Engine that is passed to this binding's observer as aEngine.

In addition, browser.js fills two lists of autodetected search engines
(browser.engines and browser.hiddenEngines) as properties of
mCurrentBrowser. Those lists contain unnamed JS objects of the form
{ uri:, title:, icon: }, and that's what the searchbar uses to determine
whether to show any "Add <EngineName>" menu items in the drop-down.

The two types of engines are currently related by their identifying
titles (the Engine object's 'name'), although that may change; see bug
335102.