I switched from Zen back to Firefox because of bugs and forced features like window sync which many users including myself don't want. I really miss having the entire UI on the side, I really don't care about Zen's other features.
Huh so if u have 2 instances of Zen open it syncs tabs between them? I don't really have that problem cos I have everything pretty well isolated in different qubes so I don't ever need multiple instances on the same machine