I’m using vertical tabs since 4 years ago and to do so installed Tree Style Tab (https://tinyurl.com/y5gr4dyn)

Also has to disable horizontal tabs create or update the file chrome/userChrome.css located at your profile with

#TabsToolbar {
  visibility: collapse;
}

and add the setting toolkit.legacyUserProfileCustomizations.stylesheets with value true (use about:config)

Top right of my tab bar in firefox, there is a little down pointing arrow. I click that: BOOM vertical tabs. Now make it an optional fixed thingie and let me stack them. I may be missing something here, but that sounds like something an experience Firefox dev can implement in half an hour.

It’s better to integrate Tree Style Tab addon instead. It’s not a good idea to re-implement a function which already has a great implementation…

I just spent 3 days learning a basic level of css and messing around in my userchrome and NOW they decide to add it…