Responsive Viewer - View multiple screen dimensions in one view
Stylebot - Change the appearance of websites instantly. Preview and install styles created by other users on stylebot.me
Responsinator - Designed to test responsive websites on different devices.
Dark Reader - Reduce eye strain in your browser with this extension that provides a dark theme for browsing.
Multi Screen Test - Test your site at different screen resolutions & devices.
Stylus - User Styles Manager - Stylus is a userstyles editor and manager based on the source code of Stylish version 1.5.2.