I don't think extensions as they are now will be able to do everything this can do. Specifically modifying the userChrome.css. Yes, there are existing vertical tab extensions, but they just reuse the existing sidebar used for the bookmarks and history. Nothing quite up to the quality that Edge or Vivaldi have.
I agree that userChrome.css must be modified, but once it is, Firefox is way better for vertical tabs. When you mix in the tree style that is common in the extensions and containers, there is nothing that competes, especially if you work involves managing a large number of accounts for the same few websites, as mine does. It is not uncommon for me to have 10-20 active tabs, and 80+ inactive tabs at any given time. Horizontal tabs can't compete, and the flat nature of the tabs in Edge certainly turn into a mess quickly.
I'm not the parent commenter, and I don't use it, but if you have a lot of tabs, it's easier to navigate between them in a vertical list if you are used to looking at the tab titles to decide which one you need.
This is partly because the tab items get very narrow when there is a lot, but also because in a vertical list there's just a lot more room for them. The top bar is ok when you know the tab you need by it's shortened title or position, but a vertical list is better when you don't and you need to search for it by it's full title or when it's further away.