Press the button once for a viewable area 800 pixels wide, and again for 1024 pixels wide. With both states, the browser height is re-sized to just below the current height of the OS desktop area. However, these defaults may changed by means of the extension's options panel.
The button must be added by the user once it has been installed by closing and restarting firefox, right clicking on the Firefox browser toolbar and selecting "Customise". The button will be one of those shown that can be dragged to anywhere on the toolbar.
This extension isn't designed for web developers who need to test their layouts at 800x600, etc (there's plenty of those elsewhere). Instead the focus is on end users who'd like to show just enough of the page they're currently reading, without it taking over the whole desktop.
I hope you find it useful.
My comments on the reviews:
Thanks for all the feedback - good or bad!
One reviewer says that you can't control the height of the browser when it is resized by the extension. I wonder if he may have missed the extension options panel (mentioned in the guide above) where this aspect may be adjusted?