I'm not seeing a ":" command bar in any demos, is there one? I use Qutebrowser and it's fantastic, I even wrote some userscripts for it which are probably easier to build than web extensions.
Currently I use Vim Vixen[0]. It's still nowhere near as feature-full as Vimperator was, but nothing can be nowadays. My only real complaint about it is how link hints don't let you complete with the link text like Vimperator could - it's the one thing that as far as I can tell, no replacement supports even though it's a whole lot better.
[0] https://addons.mozilla.org/en-US/firefox/addon/vim-vixen/