I've been using Firefox with Tab Center Reborn, and a config which collapses the tabs so only the favicons are visible. Using Shift-Tab, Ctrl-Shift-Tab and Ctrl-T, Ctrl-W to go around and open/close is so logical for me. First time I heard about Tree style tabs I was so against them because they occupied significant amount of space and I didn't know you could collapse them. I don't see myself ever going back horizontal tabs. Vertical is so much better, especially on smaller screen (like 1366x768), where horizontal space isn't needed that much as vertical is.
My setup: https://i.imgur.com/sZ8zdol.png