I tend to develop for a 960 grid system but have recently started working on an HTML5 application that I want the user to be able to resize to utilize whatever screen resolution that their display has.

To do this, I need to preview the application’s display at various resolutions.

The Firefox Resizer Plug-In is very helpful for this purpose.

It’s lets you choose what size to make the Firefox browser window.

To install it, select Tools -> Add-ons and search for Firesizer.

After you install it you still won’t see it by default.

You need to make the Add-on bar visible. View -> Tool Bars -> Add-On Bar

You should see it as in this image.

You can then right-click on the display size that is displayed.

And choose the one you want to view your page at.