You can customize the Firefox CSS, there are a lot of themes here.

Edit: I think the fennec theme is what you're after

you can easily turn window sync off. if it's not in the regular settings go to about:config and search window or sync and it will show up.

Heads up you can turn off zen's window sync in the tab management settings

Userchrome.css

Der Teckel ist mehr als ein kleiner Hund mit kurzen Beinen und langem Rücken. Hinter diesem Namen steckt eine der ältesten Hunderassen Deutschlands, ein selbstbewusster Erdhund mit jahrhundertelanger Geschichte, feiner Nase und einem Charakter, den man so kein zweites Mal findet. Während „Dackel” der Name ist, den die meisten im Alltag verwenden, sagen Jäger und der Deutsche Teckelklub von 1888 e.V. ganz selbstverständlich „Teckel”. Der Begriff verrät schon viel über die Rasse: jagdliche Wurzeln, echter Arbeitswille und ein wacher, manchmal sturer Kopf.

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.

Upvoted for Dachshund!

See the SideBery extension. The Firefox Ultima theme helps as well.

Though you still won't get the URL bar on the side, and keeping custom themes up to date is a bit of a hassle.

It's why I made the jump to Zen, you can turn off the tab synchronisation.

You can disable Window Sync on Zen.

yeah, window sync was annoying AF

I don't know of another fork, or even chromium-based one that let us put the URL bar on the side though

https://zen-browser.app/