It's a Developer extension. Clicking on Switch Env icon, will open the same URL but in the development environment that is configured. First, you have to configure in the option page the 'switch' for an specific URL and his development URL. For example: twitter.com to http://localhost:8080/some-path So, when you're in http://twitter.com/matiascarranza, click on the icon and a new tab is opened with the url http://localhost:8080/some-path/matiascarranza. Also, you can change the "localhost" to an especific URL or domain, such as 192.0.0.1 or dev.example.com The port and path options are optional.