Creating a new page with 0.00 margins or modifying an existing page margins from “Page Setup..” apparently has no effect on the margins in Firefox.

The Fix

Update these about:config entries:

print.print_margin_bottom - make it 0
print.print_margin_left - make it 0
print.print_margin_right - make it 0
print.print_margin_top - make it 0

Hope it helps!