Not to mention most websites love to waste vertical space. Navbars keep getting thicker, designers insist on having top bars even when the navigation is in the sidebar and the bottom is eaten up by cookie banners and chat/feedback popups.

I’m guessing the problem there is that mobile is far more unforgiving on horizontal space than laptops are on vertical space, so anybody building a webpage to target both ends up in the regime you’re talking about.

Good point. I suspect websites that start out as a Photoshop mock-up rather than a HTML5 prototype are more susceptible to this.

I've tried that, but you haven't lived until you've tried Tree Style Tabs in Firefox.

I like tree-style tabs, and I use it when working on projects, but I find that it is a long, long way from what it could be, which is probably a direct result of the lack of competition

the settings UI is absolutely awful;

depending on which appearance you choose, it either looks ugly or blurs together;

it frequently has visual glitches;

could certainly be more customisable appearance and shortcut-wise;

doesn’t allow you to have a title or marker as the top level tab;

as far as I can tell doesn’t let you save a session of tabs;

has an unpleasant glitch where if you drop a tab in the wrong place it’ll disappear behind the current window and never be reachable;

the new tab behaviour irritates me;

and probably a few more complaints that I’d have to actually open it up to remember

Sideberry is a worthy competitor : https://addons.mozilla.org/en-US/firefox/addon/sidebery/

I might try it next week, but if you'd mention Sideberry's advantages over TST that would be great.

Sideberry lets you turn off the "tree" feature and just have the vertical tabs.

The whole point of vertical tabs is so you have enough characters visible to tell what the tab is. Tab trees waste this precious space.

Sideberry is more complete out of the box. TST is more flexible for low level tinkering. That's what I read somewhere for TST, I use Sideberry.

As far as I can tell, Firefox, Edge and Vivaldi that are the only browsers with some way to get vertical tabs.

I don't know much about Vivaldi. Can you tell me what its monetization model is? Does it strip the google crap out of chromium?

Vivaldi was started by Jon von Tetzchner, the founder of Opera, and is meant to be a power-user’s browser with an emphasis on customizability and keyboard shortcuts, along with a range of unique features.

They do strip Google crud like FLoC, don’t pander to crypto scams and are employee-owned, you can check out their statements on Vivaldi.com. The only valid reason not to consider them would be that they are not open-source.