Mozilla released a Firefox Nightly test build with vertical tabs - gHacks Tech News
I’m using vertical tabs since 4 years ago and to do so installed Tree Style Tab (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)
@KarnaSubarna also found some interesting style snippets for Firefox
https://github.com/mbnuqw/sidebery/wiki/Firefox-Styles-Snippets-(via-userChrome.css)