Firefox dark mode or dark mode for all websites

 >Open firefox

> Click on settings

>Click on Firefox settings and Click on Dark mode

 

or You can go to themes and change the theme

or you can use extensions below:-

https://addons.mozilla.org/en-US/firefox/addon/darkreader/

https://addons.mozilla.org/en-US/firefox/addon/night-eye-dark-mode/