In Vimperator, :set toolbars=none will hide the tabs. If you mean the extra blank space at the top left by hidden tabs, then go to customize menu (:dialog customizetoolbar or burger menu next to address bar → customize) and enable Title Bar[1]. It will hide the extra space when use with toolbars=none.
[1]: https://files.grid.in.th/UHj8F1.png