In the print dialog, default paper_size is set to 'letter', despite
another paper size is being configured properly:

According to <http://www.mozilla.org/unix/customizing.html>, the
following is the correct customization:

  // Change default paper size from US-Letter to A4:
  user_pref("print.postscript.paper_size", "A4");

In the about:config dialog, 'print.postscript.paper_size' is
correctly set to "A4", which means that this value is never taken
into account by Conkeror.

Details see http://bugs.debian.org/608758.

The fact that the papersize configured in /etc/papersize or via
LC_PAPER is ignored, is though an old Mozilla/XULRunner bug and not a
problem of Conkeror. See http://bugs.debian.org/236227 and
https://bugzilla.mozilla.org/show_bug.cgi?id=147419.