The screen brightness is a Mac setting, not something that will appear in the Firefox settings page. The following websites may be helpful for you: * [https://support.apple.com/guide/mac-help/mchlp2704/10.13/mac/10.13 Change your display’s brightness] - Apple Support * [https://www.howtogeek.com/241937/how-to-adjust-your-macs-screen-brightness-manually-and-automatically/ How to Adjust Your Mac’s Screen Brightness, Manually and Automatically] - How-To Geek Hope this helps.