https://brave.com/blog/mozilla-ppa/

I agree with @trolololol@lemmy.world here. Could you describe what issues are people having? What is their user profile (programmers, writers, …)?

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)