I wrote:
Debian builds xulrunner-1.9.2.3 against the system libffi-3.0.9 and
it
just works. They even claim a minimum version of only >= 3.0.5 for the
dependency.
Actually they even claim only >= 3.0.4.
See for yourself:
http://packages.debian.org/experimental/xulrunner-1.9.2
Kevin Kofler