Change the Firefox window size to a custom size

What would you do if I asked you to test a website in the resolution 1024 x 768 for me while the native resolution of your monitor would be higher than that ? Would you reduce the resolution in your operating system to match the given figure ? There is no way to reduce the window manually to fit that resolution.

Enter  Firesizer. This Firefox extension lets you change the Firefox window size with the press of a button. Several default sizes are available which are the most used sizes websites use these days. Those are 640×480, 800×600 and 1024×768.

Additional sizes can be added to the extension and are available in the menu once they have been saved. This is a great way for  checking websites in different screen resolutions without actually having to change the resolution in Windows.