You should try the Shimmer userchrome tweaks along with the Sideberry extension. With both of them it's even better than Zen IMO.

You can set them up so they only deploy when tapping the sidebar icon and stay hidden otherwise, which is my compromise for that. I thought it'd take me longer to adapt to that when moving back from Zen, but since the top bar does deploy on proximity when using fullscreen I find it's pretty intuitive to deploy and hide them both on mouse and touch, and I have to admit that not having them deploy accidentally when hidden is actually nice.

I do like the vertical tab pins better on Firefox, and with the new tab grouping being supported on vertical tabs I am quite happy with the setup. It takes longer to set up the way I want it compared to Zen, but honestly, I'm quite happy with it now. I'd have considered going back because more alternatives is better, but frankly Zen just had too many significant bugs in my time with it, and since it doesn't just use the engine, but it's also hooked up to Firefox's account system for a bunch of stuff it just didn't seem worth the hassle. Have they polished it up any in the past few months?