Mozilla has added a Command Line for developers to Firefox Nightly and Aurora. To activate it, do the following steps:
- Go to about:config.
- Search for devtools.toolbar.enable and double-click to enable that.
- Restart the browser.
- Press Ctrl + Shift + V to open the developer toolbar with the command line.
(I suppose it will be enabled and available on the Tools menu by default in Firefox 16 – 17.)
With this command line, you can make a screenshot of a whole web page by typing:
screenshot filename.png 0 true