firefox-gnome-theme/userChrome.css

/* Here you can switch between the dark and the light theme. Don't use both, coz
 * it ain't gonna work. */

/* Dark theme */
@import "ui/theme-dark.css"; /**/
/* Light theme */
/*@import "ui/theme-light.css"; /**/

/* Symbolic tab icons */
/*@import "ui/symbolic-tab-icons.css"; /**/

/* Client-side decorations (Fedora only)
 * WARNING: This setting is experimental and doesn't work exactly as it should.
 * Don't forget to enable widget.allow-client-side-decoration in about:config
 * if you're using it. */
@import "ui/fedora-csd.css"; /**/

/* Client-side decorations window buttons options (Fedora only)*/
#nav-bar {
	/* Close window button only */
	margin-right: 44px;

	/* Two window buttons
	margin-right: 84px;
	*/
	/* Three window buttons
	margin-right: 124px;
	*/
}