If you want to test Responsive Web Design techniques against your website to make sure it accommodates various different screen sizes, then Mozilla Firefox offers a nice set of tools to allow you to do this.
- Hit the F12 button to launch Firefox's Developer Tools
- In the upper right hand corner, you will find a button for "Responsive Design Mode"
- Click on the "Responsive Design Mode" button
- You will now see a screen that allows you to select various different screen sizes for you to test your website!