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.
I must add that I find it just a little ironic, that you'd switch back to Firefox because of features Zen added, to then ask Firefox to add new features.
(This is meant as an observation, not any sort of personal attack.)