Great!

Works as stated. One little niggle is that it doesn't have the "Are you sure you want to close this window" dialog which sis a little annoying

Works great for firefox 57.0 on my debian gnome

Works great!

But, have you ever consider a 4-in-1 addon?

Don't know if it's possible, but an addon that would Hide the TitleBar AND show the 3 buttons would make a lot of sense (specially if we could which of the buttons to show).

Instead of having to install 4 addons we would need just one! =)

It will add the "X" botton to the toolbar. So we will never need tittle bar.

Simple, but useful. Means I can remove window manager's titlebar to save space and just use this extension to click and close Firefox.
Any chance of having an option to place it at the left-hand side instead? (This is where I normally place my window manager controls.)