nsIProcess
The Mozilla API for nsIProcess is not fully implemented. At present, the PID of a created process is not retained and cannot be killed. The API would also benifit from a new spec that includes inter-process communication.
Blog
A blog about the ongoing project to fix nsIProcess is available from James Boston:
http://jamesboston.ca/cms/taxonomy/term/16
Relevant bugs
https://bugzilla.mozilla.org/show_bug.cgi?id=442393
https://bugzilla.mozilla.org/show_bug.cgi?id=68702