This is the host_file_switcher add-on.

• It is a simple addon for firefox that lets you easily change your system host file.
• It works on Windows, Mac, and Linux.
• It supports sane keyboard shortcuts.
• You can select multiple host files at once.
• It allows you to have a base host file that never changes.

For more details, see the website: http://www.anthonypetty.com/hostfileswitcher