VivaldiFox - An add-on that allows colouring Firefox in a Vivaldi style fashion
Responsinator - Designed to test responsive websites on different devices.
automaticDark - Time-Based Theme Changer - Set Firefox to change the UI to a dark theme during the night and change back to a light theme during the day!
Responsive Viewer - View multiple screen dimensions in one view
Dark Reader - Reduce eye strain in your browser with this extension that provides a dark theme for browsing.
LT Browser by Lambdatest - LT Browser is a free dev friendly browser for developers and testers. You can see mobile view of website on different screen sizes and resolution on 50+ pre installed view ports. You can build, test and debug your website faster then ever.