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 3.30 theme */
@import "chrome/gnome-3.30.css"; /**/

/*******************************************************************************
 * Other features (optional)
 */

/* Hide the tab bar when only one tab is open
 * Warning: This also hides the private browsing indicator in private windows.
 * If you are using it, you might wanna enable the next feature too. Also you
 * should move the new tab button somewhere else for this to work, because by
 * default it is on the tab bar too. */
/*@import "chrome/hide-single-tab.css"; /**/

/* Limit the URL bar's autocompletion popup's width to the URL bar's width */
/*@import "chrome/matching-autocomplete-width.css"; /**/

/* Use GNOME stack icons */
/*@import "chrome/gnome-icons.css"; /**/

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

/* Import a custom stylesheet
 * Everything you add in your customChrome.css file (it doesn't exist by
 * default) will be included here and preserved between updates, so you can move
 * your configuration to that file if you don't like setting it up after every
 * update. You can also apply your own custom styles in that file. */
@import "customChrome.css"; /**/