I have a fix for this one! Took me a few minutes, but I figured it out. Works for Firefox 3.5.

1. Type about:config in the address bar.
2. Type 'header' or 'footer' (depends which one you want to edit) in the Filter box. You should now see a couple of lines like:

print.printer_(your printer).print_headerright
print.printer_(your printer).print_footerright

They have values listed at the end of the line (like &U, &PT, &D). You can edit the different lines by rightclicking on them.

You can set the default values for each printer now. I know &U stands for URL. I bet you can figure out the rest.