Bug#865650: firefox-esr FTBFS: tries to use system nspr but has no build dependency

[Adrian Bunk]

Source: firefox-esr
Version: 52.2.0esr-1
Severity: serious

https://buildd.debian.org/status/package.php?p=firefox-esr&suite=sid

...
checking for valloc in malloc.h... yes
checking for valloc in unistd.h... no
checking NSPR selection... system
checking for nspr-config... no
checking for NSPR - version >= 4.13.1... no
configure: error: you do not have NSPR installed or your version is older than 4.13.1.
...