Here's a patch for review, sets Disconnect to the default search engine, and ensures that the name "Disconnect" is used in the search box drop down menu. I confirmed that Disconnect is the default in the Omnibox, search box, and when selected text is right-clicked.
This looks fine to me. I tested and merged it for 5.0-next.
Last time you argued against using "Disconnect" in the search boxes due to concerns users could get confused (maybe they think they are disconnecting from the Tor network). I am fine with that argumentation. Could you explain what changed your mind? The "Search Search for" instead "Search Disconnect for" issue? Or was this an oversight? Or...?
The omnibox text in FF38 now says "Search" in it regardless of the selected engine. Hence, it seemed better to name the Disconnect search choice as "Disconnect", so that people can select it and not have it say "Your default engine is now 'Search'".