Update README.md

README.md

@@ -120,10 +120,13 @@ from the same directory as this README resides in, or
 
 ### Extras
 
-The `extra` directory in the same directory as this README resides in contains a Chrome/Chromium and a Plank theme.
+#### Arc Firefox theme
+A theme for Firefox is available at https://github.com/horst3180/arc-firefox-theme
 
+#### Chrome/Chromium theme
 To install the Chrome/Chromium theme go to the `extra/Chrome` folder and drag and drop the arc-theme.crx or arc-dark-theme.crx file into the Chrome/Chromium window. The source of the Chrome themes is located in the source "Chrome/arc-theme" folder.
 
+#### Plank theme
 To install the Plank theme, copy the `extra/Arc-Plank` folder to `~/.local/share/plank/themes` or to `/usr/share/plank/themes` for system-wide use.
 Now open the Plank preferences window by executing `plank --preferences` from a terminal and select `Arc-Plank` as the theme.