Hide title bar if window is maximized.
Please visit extension's page on addons.mozilla.org for install latest stable version. Also, it is good place for you positive reviews. For post bug reports and feature requests please use issues page. I read it more often.
This extension created only for GNOME 3. And it can create problems with another DE (KDE, XFCE, etc). However, you can use HTitle at your own risk.
If extension doesn't work (it's normally for DE which differ from GNOME 3) please try enable "legacy" mode in settings (on about:addons page).
Check Headerbar for Firefox tutorial for further Firefox integration with GNOME.
Maximized window without panel: