I have been using Zen as my main for over a year, and it has some random things beyond defaulting to vertical tabs (which was one of the reasons I tried it out since all of the extensions for them always felt wonky to me). Even lets you drag the window around like you do with horizontal tab bar (much easier to find free space to grab without accidentally pulling a tab into a new window for me). Some Chromium browsers also do this. So that does mess with my muscle memory when I switch over to FF and need to move the window and can't.

One kind of nice thing they have is that their version of "Peek" called "glance" kind of loads a page on top of a page without opening a whole new tab. It is kind of wonky in some links work fine, while others will just continue to the page in the original tab and need to press alt when clicking. Might be kind of a "it's just a new tab with extra steps" thing for some folks, but has been something that I have found nice to have.

Also had split tabs before they were added to FF if I remember correctly. Along with having "workspaces" that can have their own pinned tabs and and extra higher layer of pinned tabs above those. They look kind of like the boxes that are present if pinning tabs on current FF but did it first.

Outside of that, I think they managed to make the "look" of the browser better (another personal taste thing). Though FF has gotten some of the "look" closer to Zen.

Nothing "ground breaking" if you are already happy with FF. Just a pretty solid fork for people that aren't looking for something super hardened like Librewolf or even Mullvad. Early days updates had a chance of borking your GUI layout on big releases, but haven't had any issues with it in like 8 or 9 months. Overall the updates when I first started using it kind of reminded me of how early days of FF would actually excite me with obvious changes (not just GUI/UX) that felt like upgrades. This might be a non-starter for folks that prioritize GUI/UX staying more rigidly the same. Which is very valid.

vertical tabs have been a life changer, but you don't need zen for that anymore, firefox has it natively (thank allah). but i also use a 1440p monitor so im perfectly fine with losing a little screen real estate, but that + removing the bookmarks bar makes it look so slick and imho is way more intuitive since the horizontal tabs eventually clump up and you lose track of what tab is what whereas the vertical one is always clear but you can also indent groupings. highly recommend giving it an honest shot but obv you do you