My main Ubuntu desktop received an update of Firefox with the new one-click search interface a few weeks ago. I gave it a try, but I still cannot get used to the current implementation.
I previously used Ctrl+K to access the search bar and switched providers with ↑ and ↓. I thought using only ↑ to start at the end of the list when suggestions are displayed would work for selecting non-default providers, but correcting your search query and having to select the non-default provider again makes it a bit tedious.
I will try again in new releases.