When I worked at Mozilla, the official policy was that when the user overrides the default search engine, then that search engine pref is left alone.<p>(Note that if the user never changed the default, but the default itself changed, for example when Mozilla switched from Google to Yahoo (or vice versa), that was considered to be fair game.)<p>Whenever this happened while I was there, it was always due to an actual bug. I highly doubt that there is anything nefarious going on here.