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