Just the hide tabbar button would be amazing when combined with the existing vertical tab extensions. I know you can use custom css to hide the tab bar (that's what I do), but the option would make it accessible to regular users too.
"The test build shows the horizontal tab bar and the sidebar at the same time by default. A click on the new "hide tab strip" button hides the horizontal tab bar so that only the vertical sidebar remains."
I've been using "edge-style" vertical tabs (collapsed to icons by default, expanded with text on mouse hover) for the past 2-3y with a custom CSS. I think expanding on hover is the best trade off between readability and good use of space - I wish Firefox implements it some day.