Someone made an overview of the Netscape portable runtime (a little outdated, 1999) in powerpoint here
You can get the reference to the API here
You can get test cases from the Mozilla LXR. The filenames are self explanitory on what API components are tested (eg threads, locks, time). If in doubt, read the README.TXT file which lists all the files and what components they test. They are also meant as example code that is well commented.
There are several notes on developing NSPR. For example, about forks, killing threads, and porting NSPR to a new platform.
Getting in contact
The maintainers have irregular scheduled telephone meetings on Tuesdays. This requires contacting the maintainers for information.
There is also a newsgroup available here news://news.mozilla.org/netscape.public.mozilla.nspr