Heads up: Running Firefox or the rust compiler, the former using the same zone allocator registration code, the latter using jemalloc 3, get stuck after printing the following error message:

malloc: *** malloc_zone_unregister() failed for 0xaddress

This roots from the malloc zone registration code in zone.c. I'm not sure what's going on yet, and the sources for libmalloc in OSX 10.12 are not available on opensource.apple.com yet, so it's not going to be straightforward to fix.