In bug 1809002, I've finally written up some documentation of the confusing soup of macros for adding cycle collector support to your C++ class, located here:
Let me know if you have comments or questions on the documentation. I'm sure there are a lot more improvements that could be made, but it is at least a start.
There's also a new Bugzilla component for the cycle collector, Core::Cycle Collector, so you don't need to file CC bugs in XPCOM any more.
Andrew