: Under Website Appearance , select the Dark radio button. This immediately switches the browser interface and tells compatible websites to use their own dark versions.
While the native setting works for websites that already have a dark mode built-in, many sites still only offer a bright white background. To force every website into a dark theme, you can use specialized extensions available at Mozilla Add-ons: Change website appearance settings in Firefox dark mode firefox
Firefox for mobile also supports dark mode, allowing for a consistent experience across all your devices. : Under Website Appearance , select the Dark radio button