Lepton (https://github.com/black7375/Firefox-UI-Fix) to the rescue. I really hope they, or someone else, will allow us to tweak the interface and come back to what we need on our productivity oriented devices, aka computers.

I've been using Vivaldi lately. It's nice but I miss some shortcuts from firefox, and it's still Google Behind.

I feel betrayed by Firefox and depressed about the internet to come a little bit more every day.

For saving vertical space, my favorite thing about Firefox is Treestyle Tabs. I edited the CSS to make the font size a bit bigger, but make the padding so small it's slightly negative even (it would be horrible to read text like that, but for tabs it kinda works). Now I can fit ~40 tabs vertically on the screen easily, with very readable tab titles, and I can expand the sidebar at will if I want to see the full length of titles for some reason, or use F1 to toggle it on and off.

Not that this takes away from your point.. but frankly, as long as the browser UI can be customized, the default setup will never matter to me. If this appeals more to non-technical people, more power to them, as long as I can have my power user stuff, too.

Useless padding and borders seems to be par for the course in the last couple of years. Another thing redesigned for no tangible benefit.

What part of Zen's UX does Firefox need to learn from?

I used Zen for a while (and Arc before that). I didn't get what was particularly novel about either. My browsing did not feel revolutionized.

What have I not been getting?

Edit: I realize it may not come across as one but this is a genuine question.