For those who often find themselves facing way too many tabs on their tab bar, Vivaldi 3.6 comes with a second one. In addition to two-level tab stacks, the latest update for this browser also includes split-screen view and tree-style view (via the Tab Tiling and Window Panel features).
https://www.notebookcheck.net/Vivaldi-3-6-now-available-with-two-level-tab-stacks.517421.0.html
The split-screen looks cool and should come in really handy, and the tab stacking change is an improvement, but Vivaldi really needs to add multi-row tabs already. This is the only real obstacle to a lot of users transitioning over from Mozilla-based browsers. If they added that and a real status bar that you could move add-on controls to, I would switch over.