I switched from Zen back to Firefox because of bugs and forced features like window sync which many users including myself don't want. I really miss having the entire UI on the side, I really don't care about Zen's other features.
Vivaldi just had a nice update with an auto-hide feature.
I am a fan of vertical tabs but usually it takes up a little too much left hand space for my liking so I'd semi-frequently be adjusting the size depending on if I needed it, now with the recent change all of it gets out if your way until you hover over there, I'm liking it a lot so far.
Both Chrome and Firefox's vertical tabs are so much worse, Edge's somehow the best out of the mainstream browsers imo.