firefox-gnome-theme/theme/gnome-theme.css

@import "parts/toolbox.css";
@import "parts/headerbar.css";
@import "parts/csd.css";
@import "parts/popups.css";
@import "parts/tabsbar.css";
@import "parts/remove-white-flash.css";
@import "parts/custom-icons.css";
@import "colors/light.css";
@import "colors/dark.css";
@namespace xul "http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul";

*{
	font-family: Cantarell !important;   
}

:root {
    --space-above-tabbar: 0 !important;
}

#main-window {
  -moz-appearance: -moz-window-titlebar !important;
}