/usr/lib/x86_64-linux-gnu/pkgconfig/xulrunner-nss.pc is in libnss3-dev 2:3.35-2ubuntu2.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/lib/x86_64-linux-gnu/pkgconfig/nss.pc
/usr/lib/x86_64-linux-gnu/pkgconfig/nss.pc