It was added for nspr and nss back in the 55.0.3 to 56.0 upgrade. It
also served as a workaround for an undeclared gio-unix-2.0 dependency.
Sometime afterwards nspr was removed, leaving just the two. Since then,
upstream has added a declaration for gio-unix-2.0 (in FF62). As for the
nss include it seemingly has no purpose since current firefox builds
with it removed.