We'll need to treat search engines slightly different from regular extensions. - We may want them in a different tab like themes - We don't want certain engines to appear at all (see bug 1491072) - We may want to strictly limit engines to having only the search_provider in manifest (no background/browser/page/sidebar/etc) Whether a new type is necessary or not may be questioned, but at least the second point above must be addressed.

Bug 1365349 adds a way for system addons to hide themselves from about:addons.