There was another add-on called Toolbar Autohide that used to hide the Toolbar + Addressbar and worked like a charm.

You could set the sensitivity, height and other tweaks to control the collapsing of the hidden bars.

Using undecorated windows, that left me with nothing but a window of the web page. No addressbar, no tab bar but available at a quick mouseover.

I hope there's some way of achieving this in FF Quantum.

I already have the TST plugin with the F1 key toggle and hidden tab bar.