Click the menu button, click Add-ons and select Themes. Firefox will download the requested theme and may notify you that it has been installed.
To install a theme from, click on the theme and select the + Install Theme button on the resulting page. You’ll instantly see the Dark Theme mode with dark backgrounds, toolbars, and tabs. 3) At the very bottom, click the Themes button. 1) Click the Open Menu (three-line icon) button from the top right. Open Firefox and then follow these steps to enable the Dark Theme.