As mentioned earlier, it is really important to thoroughly test if your application is design responsive. Earlier, developers had to take care of this task by resizing their browser windows, but thankfully, Firefox offers Responsive Design View in its arsenal of developer tools. You can go over to responsive design view by navigating to Tools | Web Developer | Responsive Design View. This allows you to check your applications by testing them in some preset sizes of mobile devices. You can also set a custom size by dragging the corners of the mobile responsive view to test the elements. It allows other options, such as displaying the page in landscape mode at the device size that you have set and simulating touch events. You can also take a screenshot of the application in the size that has been set by you by clicking on the camera icon.
The following is an illustration of our application Clickr in Responsive Design Mode set at size 320*480...