Zen (Firefox-based) has been really refreshing. You could probably accomplish the same thing with some user scripts and user CSS, but the concern with those has always been that they could break at any time with a new update. That shouldn't happen with a fork like Zen as they have control over updates.
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.
