Linux Ubuntu 14.04.3 LTS screen preview with top bar [1]..
Open the Mozilla FireFox program and enter the URL:
https://extensions.gnome.org/extension/545/hide-top-bar/
On the screen below. Press the option "Click here to install the browser extension" (1). This extension is required to control the GNOME Shell.
With the GNOME Shell installation complete. Refresh the page.
On the next screen, the ON / OFF button (1) will be activated.
Click the button to enable the ON option (1).
With the button enabled. A message will appear with the option to install the Hide Top Bar. Click the Install button (1).
Listo. Hide Top Bar instalada correctamente.
Linux screen preview with top bar (1) hidden.