It seems weird for this to be non-fatal.
I was hitting the locally because I made a copy-paste error where I declared refcounting with the name of another class.
Created attachment 8775204 [details] [diff] [review] Mismatched sizes assertion in nsTraceRefcnt.cpp should be fatal. try: https://treeherder.mozilla.org/#/jobs?repo=try&revision=de6442645240
Pushed by email@example.com: https://hg.mozilla.org/integration/mozilla-inbound/rev/bd5e978762ce Mismatched sizes assertion in nsTraceRefcnt.cpp should be fatal. r=froydnj