An integrated experience. In the past I found that the vertical tab options in Firefox had the tabs duplicated across the side and the top, which I always found to be a subpar experience. Again, probably something you could accomplish with user.js and user.css but there's a good chance an update could break your modifications.
Seems quite similar to Zen's experience, except it seems to be missing folders (which I admittedly don't use often, but they're sometimes handy to group a Jira ticket with a PR, or similar). I'll probably still stick with Zen while it's around, and maybe I'll hop over to LibreWolf as I'm not too happy about Mozilla's recent stance on privacy.
