It always bothers me about this whole vertical tab concept, that, on a theoretical level, I'm fully on board with what you're saying. But in practice, I'll often have Firefox tiled side-by-side with another window, and then it's painful for that sidebar to take up any space at all.

I am happy, though, that this feature is being integrated for the people that find it useful.

But how are you going to read the title of the tab without making the sidebar too wide? That's like having a vertical taskbar in the days before Windows 7 came out.

Monitors are horizontal so we can have two windows opened side by side, making both squares.

You also read page titles horizontally, making it easy to skim tab page titles.

Not if you need to see all those tabs you're hoarding.

Exactly. Monitors are horizontal. Tab names are horizontal. They should be listed vertically with the names written horizontally. You can hide them if you need to, but I find my monitor has plenty of space.

That would work well with my dual 32" 4K monitors

I'm not so sure it would work well on a 15" laptop. It's easier to scroll vertically than horizontally

... you can hide them and only show when you open a new tab ...

Not like you need your bookmarks at the same time while browsing other pages.