Hi!
Different arch flavors of libnspr4-dev can not be co-installed.
Please add the Multi-Arch: same  hints to the debian/control file
Also, nspr-config - which is a shell script - contains the library path -
making it different.

if test -z "$libdir"; then
    libdir=/usr/lib/x86_64-linux-gnu
fi

This can be dynamically queried from dpkg-architecture -qDEB_HOST_MULTIARCH

Much appreciated.
Thank you