I've been running into more and more bugs with Zen, often around tab management.

• The other day I tried moving some tabs between windows and the tabs just disappeared. They were still accessible with CTRL+tab but otherwise hidden.
• It seems to make new tabs randomly when closing other tabs.
• The workspaces are confusing and linked to individual browser windows. If you move out of a workspace, you can't get it back unless you open it in the same window you left it from.

Might switch back to Firefox and wait for more development, or Firefox's native vertical tabs.

Cool experiment nonetheless