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.
This is a FEATURE? I've been frustrated with this "bug" for WEEKS whenever I needed a clean auxiliary browser window with only a tab or two and the hundreds of tabs from the main window got duplicated in the new one making it equally hard to navigate. Sheez.