The choice which engines to sync (engine.enabled) should affect all clients, not just the one you're making the choice on. Once a particular engine is disabled, its data should also be wiped on the server. Engine.enabled is persisted as a preference, but since the preference engine could be disabled just as well, we can't really use it to sync that info. The meta/global WBO seems like a good place (inside the 'engines' hash) since it's checked before every sync anyway.

Bug 563459 is probably related.

Yes, syncing which engines are enabled implies prepopulating those checkboxes.

Observe and remember when the engine enabled prefs change. Upon the next sync, check for newly enabled or disabled engines both remotely and locally. Presence in meta/global's engines object is interpreted as being enabled.

Change strings to indicate that the engine setting is no longer contained to each individual computer.

http://hg.mozilla.org/services/fx-sync/rev/5571f5038b93 http://hg.mozilla.org/services/fx-sync/rev/9bd8a5592b2f

I was recently playing with this on Minefield and noticed that I didn't get any message saying whether pre-existing data on the server will be deleted or if it will remain there? I think we should let users know what happens as it is not immediately obvious.

Please file a followup on that and we can discuss it in that bug?

Filed: https://bugzilla.mozilla.org/show_bug.cgi?id=607407