I've seen it. Sadly it's just a nice thin interface over WebKit. I guess I could customize it to do somewhat what I want, but it would still run on WebKit. On Linux I can use alternative browsers like links -g, WebSurf or Dillo, but on mobile there are no such luxuries.
I would just like to have the target experience (and lightness) out of the box, but on all platforms.
https://surf.suckless.org/
It's webkit, not firefox-based, but works great with tiling windowmanagers.