It has a few privacy features, some themes, some other stuff. Nothing significant. It kinda became popular lately, and some people started using it. But now it's proprietary, so I wouldn't use it anymore. LibreWolf is much better and open source.
It also has double sidebars so I can put tabs in one of them instead of the top of the screen and hide the titlebar without having to modify userchrome.css. That's one thing I missed from Vivaldi when I moved to Firefox.
Yeah, Vivaldi’s ability to make itself surprisingly minimal (in a clean/non-“hacky” way) is the only thing keeping me from Firefox or Librewolf right now.
If you want you can try Floorp if you'd like to use Firefox based browser. It is really customizable, although it is missing a lot of things that Vivaldi has, like split screen and tab grouping. But it has an enhanced support for tree style tabs which is a big plus in my opinion.
Yeah I’ve poked at Floorp but it’s UI mods feel kinda janky at this stage. I really like being able to turn everything off (no tab bar, address bar only when activated), and the only other browser that can do that is Orion (or maybe qutebrowser? No extensions though :/)
I believe the main thing people liked about Floorp is tab grouping and vertical tab layout à la Vivaldi, and a more modern and slim design out of the box, while keeping a firefox core instead of being another chromium based browser.
Like said below, it's nothing too significant. It boasts about having two sidebars on each side and "Flexible Toolbar & Tab Bar", which according to the screenshots is TreeStyleTab + another toolbar on the right. In my opinion, that is something different. And that's what the website says.