Sorry, Firefox doesn't support multiple lines in either the header or footer of printed pages.

In about:config -- typed in the URL bar and hit Enter You can filter by print.print_footer in the top Search field. look at: print.print_footerright print.print_footerleft print.print_footercenter for the String Values which are set thru Page Setup. I have never tried to change those values via about:config - but I think changing them there is possible.

As far as any other values as used in Internet Explorer, it's been so long since I fiddled with headers / footers that I don't recall what's possible. When I viewed the Firefox source code for the printer functionality back in 2004 - 2006 a few times. Problem with later revisions seems to me to be that the Mozilla developers don't provide comments or annotate their code anywhere as thoroughly as back in the days of Netscape, if they provide any comments at all. But I do recall it is somewhat limited and less than can be done with IE 15 years ago.

Does \A or \n has any effect?

Thank you the-edmeister and cor-el. Btw \A and \n do not work