I have tried all the Vertical Tabs extensions post-FF59 I could find, and this is definitely the best!
Simple, fast, does not mess with the opening and closing ordering of tabs and focus change, allows drag & drop of tabs between windows.

The speed gain on all FF tab operations compared to others extensions such as "Power Tabs" is really noticeable.

The only missing feature is a way to "detach" a tab into a new window (I have to use the tab on the top of the window, or create a new window and drag+drop the tag there), unless I have missed something.

Tab Sidebarシンプルであること、それがとても良いです。
Default Style Sheetを見ることができるのもうれしい。
サイドバーに表示されたタブをマウスホイールで切り替えることもオプションで可能となりました。
ありがとうございます。

Exactly what I was looking for.