Firefox for iOS can automatically switch to a dark or light theme to match your device’s settings if you have iOS 13. See this Apple support article for additional information regarding Dark Mode.

Automatically match system theme

When you switch your device to light or dark theme, Firefox will match your system theme by default. To change this:

1. At the bottom of the screen, tap the menu button (the menu will be at the top right if you're using an iPad):
   

2. Tap Settings in the menu panel.
3. Tap Theme.
4. Tap the Use System Light/Dark Mode toggle (blue means it’s on).

Manually or automatically set themes

You have the ability to manually or automatically set themes

1. At the bottom of the screen, tap the menu button (the menu will be at the top right if you're using an iPad):
   

2. Tap Settings in the menu panel.
3. Tap Theme.
4. Tap the Use System Light/Dark Mode toggle to turn it off (gray means it’s off).
5. Tap an option from the Switch Mode menu:
   • Manually: all theme changes will occur manually.
   • Automatically: theme will match your screen brightness
6. Tap a theme from the Theme Picker menu:
   • Light
   • Dark
7. Customize your theme’s brightness using the Threshold slider.