Or Vivaldi natively.

Brave is adding vertical tabs. It's currently available via flag on Nightly and Dev. Hope they add nested tabs also!

Thanks for that info! I think nested tab are lame as it wastes some of the tab title length. I use a browser window for each high level topic, which mimics the advantages of high level nests.

This is a fair point. In recent years (before Brave supported tabs, but after I left Firefox), I had to use a single window because the TST-style tab manager sat beside it in a separate window. This was annoying enough even with a single browser window, and it was unmanageable with multiple windows.

But now that I'm using native tabs, it is much easier for me to use multiple windows because the tabs are attached. Thanks for pointing this out!

Just to add another suggestion to the multiple windows approach: the Titler extension allows you to name each window so that the title shows up before the current tab name of that window. Unfortunately, Sidebery doesn't pick it up, which makes moving tabs between windows a bit of a pain...

IMO Firefox's open tabs menu is a pretty good stand-in for vertical tabs.

Vivaldi also has vertical tabs out of the box, with single-level grouping - very similar to Edge.