firefox-gnome-theme/userChrome.css

/*
 * In this file you can enable styles which apply to Firefox' user interface.
 * To enable a specific feature, uncomment its @import line by removing "/*" at
 * the beginning of the line. To disable it, put the "/*" back.
 */

/*******************************************************************************
 * GNOME Theme
 * Pick whichever variant you like, it doesn't have to match your GNOME version,
 * but it will probably look better if it does. Remember that your GTK theme
 * variant must match the variant you pick here, eg. if you pick a light
 * variant, you must also run Firefox using the light variant by either
 * disabling global dark theme in GNOME Tweak Tool or running Firefox explicity
 * with the light theme:
 *   $ GTK_THEME=Adwaita:light firefox
 * The opposite applies to the dark variant, you must either enable global dark
 * theme or run Firefox like this:
 *   $ GTK_THEME=Adwaita:dark firefox
 */

/* GNOME 3.18 light theme */
/*@import "ui/gnome-3.18-light.css"; /**/
/* GNOME 3.18 dark theme */
/*@import "ui/gnome-3.18-dark.css"; /**/

/*******************************************************************************
 * Optional features
 */

/* Make all tab icons look kinda like symbolic icons */
/*@import "ui/symbolic-tab-icons.css"; /**/

/* Enable 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"; /**/