/usr/include/firefox-esr-52/nspr/ is in firefox-esr-dev 52.8.1esr-1~deb8u1.
This file is owned by root:root, with mode 0o755.
..
/usr/include/firefox-esr-52/nspr/md/
/usr/include/firefox-esr-52/nspr/md/_darwin.cfg
/usr/include/firefox-esr-52/nspr/md/_freebsd.cfg
/usr/include/firefox-esr-52/nspr/md/_linux.cfg
/usr/include/firefox-esr-52/nspr/md/_netbsd.cfg
/usr/include/firefox-esr-52/nspr/md/_openbsd.cfg
/usr/include/firefox-esr-52/nspr/md/_win95.cfg
/usr/include/firefox-esr-52/nspr/nspr.h
/usr/include/firefox-esr-52/nspr/plarena.h
/usr/include/firefox-esr-52/nspr/plarenas.h
/usr/include/firefox-esr-52/nspr/plbase64.h
/usr/include/firefox-esr-52/nspr/plerror.h
/usr/include/firefox-esr-52/nspr/plgetopt.h
/usr/include/firefox-esr-52/nspr/plhash.h
/usr/include/firefox-esr-52/nspr/plstr.h
/usr/include/firefox-esr-52/nspr/pratom.h
/usr/include/firefox-esr-52/nspr/prbit.h
/usr/include/firefox-esr-52/nspr/prclist.h
/usr/include/firefox-esr-52/nspr/prcmon.h
/usr/include/firefox-esr-52/nspr/prcountr.h
/usr/include/firefox-esr-52/nspr/prcpucfg.h
/usr/include/firefox-esr-52/nspr/prcvar.h
/usr/include/firefox-esr-52/nspr/prdtoa.h
/usr/include/firefox-esr-52/nspr/prenv.h
/usr/include/firefox-esr-52/nspr/prerr.h
/usr/include/firefox-esr-52/nspr/prerror.h
/usr/include/firefox-esr-52/nspr/prinet.h
/usr/include/firefox-esr-52/nspr/prinit.h
/usr/include/firefox-esr-52/nspr/prinrval.h
/usr/include/firefox-esr-52/nspr/prio.h
/usr/include/firefox-esr-52/nspr/pripcsem.h
/usr/include/firefox-esr-52/nspr/private/
/usr/include/firefox-esr-52/nspr/private/pprio.h
/usr/include/firefox-esr-52/nspr/private/pprthred.h
/usr/include/firefox-esr-52/nspr/private/prpriv.h
/usr/include/firefox-esr-52/nspr/prlink.h
/usr/include/firefox-esr-52/nspr/prlock.h
/usr/include/firefox-esr-52/nspr/prlog.h
/usr/include/firefox-esr-52/nspr/prlong.h
/usr/include/firefox-esr-52/nspr/prmem.h
/usr/include/firefox-esr-52/nspr/prmon.h
/usr/include/firefox-esr-52/nspr/prmwait.h
/usr/include/firefox-esr-52/nspr/prnetdb.h
/usr/include/firefox-esr-52/nspr/prolock.h
/usr/include/firefox-esr-52/nspr/prpdce.h
/usr/include/firefox-esr-52/nspr/prprf.h
/usr/include/firefox-esr-52/nspr/prproces.h
/usr/include/firefox-esr-52/nspr/prrng.h
/usr/include/firefox-esr-52/nspr/prrwlock.h
/usr/include/firefox-esr-52/nspr/prshm.h
/usr/include/firefox-esr-52/nspr/prshma.h
/usr/include/firefox-esr-52/nspr/prsystem.h
/usr/include/firefox-esr-52/nspr/prthread.h
/usr/include/firefox-esr-52/nspr/prtime.h
/usr/include/firefox-esr-52/nspr/prtpool.h
/usr/include/firefox-esr-52/nspr/prtrace.h
/usr/include/firefox-esr-52/nspr/prtypes.h
/usr/include/firefox-esr-52/nspr/prvrsion.h
/usr/include/firefox-esr-52/nspr/prwin16.h