The cycle collector has a Fault method that does NS_NOTREACHED, but some of the places this can happen are pretty scary, so we should crash the browser instead of silently shutting down the CC and continuing onwards. I'd probably want to land this early next cycle.
bsmedberg gave me an r+ over IRC to do this, I just never did it.
Do you have a bug and/or patch for it?
https://bugzilla.mozilla.org/show_bug.cgi?id=521853#c4 I didn't write a patch, but it should be easy.