To start, you must have Firefox, then you have to install the FireForm extension from Firefox Add-ons.
You now have a new icon in the status bar, and a new fireform menu in the tools menu. Go to this tools menu, choose profile management and you get this window.
Just fill in the fields you like and erase others.
Now its done. Clicking on the button will fill the current page.
A right click also allows to change profile.