(the download will begin shortly. If the download does not start, try this direct link.)
The Browser Window Resizer, a Firefox extension, is useful for testing different screen sizes. It accurately resizes your browser's window so you can test to see what a web page looks like in all of the standard resolution sizes.
Supports the 640x480, 800x600, 1024x768, 1280x1024, 1600x1200 resolutions.
Resize the current page via the Context menu, Tool menu or Toolbar button.