The following seems related and has more kudos: https://connect.mozilla.org/t5/ideas/ability-to-change-default-profile-in-the-profile-manager/idi-p/85137
I am not sure whether this should be a feature request, or may be considered a bug, as I believe the previously default profile system did handle this feature. As such, it perhaps it would be considered a bug that it is missing in the new system?
