Have you tried Print > Page Setup > check "Shrink to fit"?
Check the selected printer Name: Print > Print
Check your margins: Print > Page Setup > Margins & Header/Footer

Also see: http://kb.mozillazine.org/Problems_printing_web_pages

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Three possible causes.

1. The page is made with Frames and each Frame is printing separately. If you right click the page you would see a menu item for This Frame , if that page has Frames. You'll see that on this test page.
http://www.draac.com/framestest.html

2. A wide page margin setting. You might be able to see that in Page Setup > Margins/Header&Footer, but some printers manage to get weird settings that don't show up there. When you look in about:config these prefs are way off
print.printer_printer-name_Printer.print_margin_left
and
print.printer_printer-name_Printer.print_margin_right

3. A non-standard Paper Size is set in Properties for that printer as viewed through Firefox. Might set to Custom or to a Photo size.

See this section of support article hyperlinked by SafeBrowser.
http://kb.mozillazine.org/Problems_printing_web_pages#Prints_to_a_small_portion_of_the_page

For #2 and 3, doing a reset should solve it. Do the 'reset as described here:
http://kb.mozillazine.org/Problems_printing_web_pages#Reset_printer
Make sure you reset all the prefs for that particular printer.

I think I may have solved the problem -- I had a couple of plug-ins that needed to be updated. I did that and now it seems the printing issue has solved itself.