install_hide_firefox_titlebar.sh

#!/bin/bash
# date: 2017-11-17
# license: GPLv3 https://www.gnu.org/licenses/gpl-3.0.txt
# author: nanpuyue <nanpuyue@gmail.com> https://blog.nanpuyue.com

echo 'install: ~/.mozilla/native-messaging-hosts/hide_titlebar.json'
[[ -d ~/.mozilla/native-messaging-hosts ]] || mkdir -p ~/.mozilla/native-messaging-hosts
cat > ~/.mozilla/native-messaging-hosts/hide_titlebar.json << EOF
{
    "name": "hide_titlebar",
    "description": "Hide the Firefox titlebar.",
    "path": "$HOME/.local/bin/hide-firefox-titlebar.sh",
    "type": "stdio",
    "allowed_extensions": ["hide-titlebar@nanpuyue.com"]
}
EOF

echo 'install: ~/.local/bin/hide-firefox-titlebar.sh'
[[ -d ~/.local/bin ]] || mkdir -p ~/.local/bin
cat > ~/.local/bin/hide-firefox-titlebar.sh << "EOF"
#!/bin/bash
# date: 2017-11-17
# license: GPLv3 https://www.gnu.org/licenses/gpl-3.0.txt
# author: nanpuyue <nanpuyue@gmail.com> https://blog.nanpuyue.com

python3 -c "import gi; gi.require_version('Gdk', '3.0'); from gi.repository import Gdk, GdkX11;
default_display=GdkX11.X11Display.get_default();
$(for i in $(xdotool search --onlyvisible --class "Firefox|Nightly"); do
    echo "GdkX11.X11Window.foreign_new_for_display(default_display, $i).set_decorations(Gdk.WMDecoration.BORDER);"
done)
Gdk.Window.process_all_updates();"
EOF
chmod +x ~/.local/bin/hide-firefox-titlebar.sh

echo 'done'

PSA: Anyone looking for this feature, before trying this script check if you have the option to disable the title bar down in the lower left corner of the firefox customization screen.

Right click Menu Button > Customize > Uncheck "Title Bar"

Thanks, man! you`r a firefox's god

There is a problem with this solution -- it removes bars from all ff windows you have open, including other profiles which may not have this extension and thus deleting is unwanted

I don't remember the last time I used to need this. Modern Firefox can apply that from "Personalize taskbar" option, which is called "hide taskbar". It works as expected.