Hey, I'm with you in feeling that tabs are essential to web browsing but that they should ideally be the window manager's responsibility. For a while I used surf [1] (which lacks tabs) and the i3 window manager, but for various reasons I wanted to move back to Firefox.

I made a very simple webextension that disables tabs in firefox, so that I can use my window manager's tabs instead. I've been using it since the stable release of Firefox 57. http://static.adrusi.com/notable.html

[1] https://surf.suckless.org/