Responsinator - Designed to test responsive websites on different devices.
VivaldiFox - An add-on that allows colouring Firefox in a Vivaldi style fashion
Responsive Viewer - View multiple screen dimensions in one view
ShadowFox - This project aims at creating a universal dark theme for Firefox while adhering to the modern...
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.
Firefox Color - Build Beautiful Firefox Themes