How do I remove the header and footer generated by Firefox?

Click the Firefox menu (the one with 3 horizontal bars)

Pick Print, you'll see a preview of how the page will look.

At the top of the screen, click Page Setup

and then click onto the "Margins & Header/Footer" tab. 

Look for the drop-down box which says "Date/Time" or "URL" or whatever is setup for the Header and Footer and change it to "--None--".