Natively testing responsive layout on Firefox

If you want to test you responsive layout in Firefox and don't want to install a extension or some bizarre script, just go to Tools -> Web Development -> Responsive Design View or simply Ctrl (or command on OSX) + Shift (or alt on OSX) + M.

It will give you a resizable screen inside the browser window and some default breakpoints (you can add yours).

This can speed up the responsive design development a lot!