We include all search engines by default, and enable them per list.json[1]. We need to update this to - point at xpi instead of xml - enable/disable engine extensions as necessary System extensions are typically enabled/disabled via pref in a ad-hoc fashion. Bug 1489527 aims to resolve that, so we may make use of this, but that is yet to be determined. We may just enable/disable directly from the search service as necessary. [1] https://searchfox.org/mozilla-central/rev/a23c3959b62cd3f616435e02810988ef5bac9031/browser/components/search/searchplugins/list.json