Using --enable-threadsafe and --with-system-nspr I get:
The workers seem to get
Assertion failure: rt->onOwnerThread(), at /moz/mi-to-commit/js/src/jsapi.cpp:6302
which I believe luke recently added. regress-422269 is a leak test that gets screwed up by conservative stack scanning sometimes.
Hopefully threadsafe and the conservative stack scanner are both going away.
Shell workers are gone, so this should be all fixed.