I am not aware of anyone ever trying to implement it. There is not even a plugin from what I know. If someone would integrate the UI in a plugin, that works and

I am not aware of anyone ever trying to implement it. There is not even a plugin from what I know. If someone would integrate the UI in a plugin, that works and people use it, I would be surprised if Mozilla would not accept a proper Patch to Firefox implementing that. The proposed UI was not blocking anything, not even taking space. Given Persona survives this.