
Thanks to Shin'ya Ueoka, there's Vim Vixen, which brings some of the vim(perator) feeling back to the current Firefox.
Since Vim Vixen doesn't hide the navigation toolbar by default, here's a short "HowTo for auto-hiding the navigation toolbar"

This is done by using a custom "userChrome.css" to hide/show toolbars.
Tested with Firefox Quantum 64.0 (on Xubuntu 16.04.1).
1) Download custom CSS for hiding the toolbar:
Tim Van den Eynde was so nice to put together collection of CSS files for userChrome mods, called: "UserChrome Tweaks".
One file there (by Alex Vallat?) does exactly what we need!
It's called: toolbars/auto-hide.css
2) Save it as "userChrome.css":
In your firefox profile folder (e.g. ~/.mozilla/firefox/asdfasdf.default/), create the subfolder "chrome".
Then save the "auto-hide.css" there and rename it to "userChrome.css" (Please preserve the uppercase "C").
3) Restart Firefox.
If everything is right, your navigation toolbar should now be hidden when opening firefox: It will show up when you hover your mouse over it though

Have fun!