The problem is easy enough to diagnose. In previous versions of xulrunner, these headers were shipped as part of xulrunner-dev, so they were in /include/xulrunner-[version]/stable. Now, for whatever reason, these headers are not included in this package. Unless this is a mistake, it looks like these two programs will have to add include flags for package nspr-dev in addition to xulrunner-dev.
The problem is easy enough to diagnose. In previous versions of xulrunner, these headers were shipped as part of xulrunner-dev, so they were in /include/ xulrunner- [version] /stable. Now, for whatever reason, these headers are not included in this package. Unless this is a mistake, it looks like these two programs will have to add include flags for package nspr-dev in addition to xulrunner-dev.