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