I tried Zen for a while (1-2 months). It certainly has some cool features. But in the end, I returned to Vanilla FF and installed Sideberry [0] with custom userChrome.css [1]. It gets me 99% of what I used with Zen (tree tabs, workspaces), but without annoyances/anti-features (inconvenient url bar for editing, frequent UI changes, cute animations that ignore prefer-reduced-motion, performance, security worries etc.).
I’m relatively happy with my setup [2] now, what I miss most from Zen would be the 2-level pinned tabs (pinned per workspace, and globally pinned), and the design of globals pins (instead of a line on the side as in [2], it’s a grid at the top for Zen), but not even close to enough that I’d want to return.
[0]: https://addons.mozilla.org/en-US/firefox/addon/sidebery/
[1]: https://github.com/mbnuqw/sidebery/wiki/Firefox-Styles-Snipp...