One of the outstanding feature of the latest version of Mozilla had been Responsive Design View which had made my life as web developer very very easy.
Basically this tool allows user to test the website appearance on different browser size. Its much more convenient than resizing the browser size now and then. Its easy as well as the other tabs opened on the browser wouldn’t get affected.
How to use the Responsive Design View
Step 1 : Hit Alt + T
Step 2 : From Web Developer View Select Responsive Design View
Step 3 : Select the custom size or add a preset
Its as easy as that. A more details can be found at mozilla’s site.