You can disable the horizontal tabs on the title bar by adding the following line to userChrome.css in the Chrome directory of your Firefox profile: #tabbrowser-tabs { visibility: collapse !important; }

If you don't have the Chrome directory and userChrome.css, make them. Go here for information about how to access your firefox profile: https://support.mozilla.org/en-US/kb/profiles-where-firefox-stores-user-data

I hope Piro-san will solve this in a smarter way.
And Thank you so much for your hard work. This extension is the reason why I never switched to other browser.