Right now, we just include two entries for browser.search.order by default. When a new engine is added nothing is updated in the prefs. Ideally, we need to add new entries each time an engine is added to reflect the order of the search engines. Steps to reproduce: 1. Add a new search engine. 2. Check in about:config under browser.search.order.* There should be a new entry there reflecting where the new search engine was added.
Why should there be a pref added there? It won't really do anything, and there is no UI exposed for these prefs, so I don't see how it makes a difference.
Because of the way ordering was implemented, the pref is added now at shutdown.