Took a little finnagling to get FF 10.0 under OS X to work, so for any other mac users out there:
To change your margins, you'll want to go File -> Page Setup, and the 3rd dropdown (Paper Size), choose Manage Custom Sizes. There you can create a new custom size (I made a 8 1/2 x 11 inch paper with .1 margins all the way around).
To fix the problem where none of hte colors and almost none of hte icons show in a PDF/printed version, hit cmd-P (or choose print from the File Menu), and Click Show Details.
The dropdown menu at the top should say "Firefox" but if it doesn't, select it, and check the 2 boxes "Print Background Colors" and "Print Background Images". You also probably want to remove the header and footers there. then the PDF menu on the lower left will let you print your sheet to PDF, if you're not printing to an actual printer.
Nice tool, I'm excited to try it out!