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.
See the SideBery extension. The Firefox Ultima theme helps as well.
Though you still won't get the URL bar on the side, and keeping custom themes up to date is a bit of a hassle.
It's why I made the jump to Zen, you can turn off the tab synchronisation.