>- Vertical/tree-style tabs:
https://addons.mozilla.org/en-GB/firefox/addon/sidebery/This is the biggest reason why Chrome sucks - they have no workable version of vertical tabs.
Reasons why horizontal tabs are clearly inferior:
1. Screens (generally) have way more horizontal space than vertical and you read webpages vertically. I already have an OS top bar and a browser top bar taking some of this valuable vertical space.
2. Most webpages have predefined right and left margins, so vertical tabs don't take away from the visible space of the page.
3. The size of the tab header is critical for being able to read the name of the tab. I have my vertical tabs set to 30 characters. If I had horizontal tabs with 30 characters, I could fit maybe 6 tabs on the screen at once.
4. Ease of having many more concurrently open tabs. I easily have 38 tabs open and visible without scrolling. 8 of those are pinned tabs which take up the space of one tab. I use 6 different windows (which are category titled using the Titler extension). That is 228 tabs open at once and very easily accessible and readable. (of course I use Total Suspender to keep them from taking too many resources)
All this said, Firefox still sucks because vertical should be the default or at least a simple toggle. As it is, you have to trust some extension and then you have to jump through obnoxious hoops just to get the damn horizontal tabs turned off. And yes, there are some other browsers that have vertical tabs built in, but I am not touching Edge or Opera as they are from dubious companies. I also think that Brave is working on it, but they seem dubious as well.
Now to my complaints about Sidebery (which is superior to Tree Style Tabs). Sidebery has tree style as well, but I turn it off as it undermines the value of #3 above. It has panels, which are goofy and, while you don't have to use them, you still lose a tab worth of space for it. You also lose an even larger tab space to the Sidebery title (which is a menu and an X that aren't necessary. Sidebery doesn't work well with the Titler extension - when you want to move a tab to another window, it doesn't show you the title.