Firefox has an interesting feature called the
Developer Toolbar. This
toolbar is entirely separate from the JS console and provides a cli for custom
commands for specific functionality. It can opened with
Developers > Developer Toolbar.
In that toolbar, type:
screenshot --fullpage my-page.png
A png file with the given name will be saved to the Downloads folder. The file format will always be png.
Developer Toolbar information can be found here.
help in the toolbar to see all the commands.