/usr/lib/x86_64-linux-gnu/pkgconfig/xulrunner-nspr.pc is in libnspr4-dev 2:4.18-1ubuntu1.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/lib/x86_64-linux-gnu/pkgconfig/nspr.pc
/usr/lib/x86_64-linux-gnu/pkgconfig/nspr.pc