Gecko has long had assertions to verify that XPCOM objects are AddRefed/Released on the right thread. Today I landed Bug 753659 which makes those assertions fatal (using MOZ_ASSERT). This makes these assertions noticeable on test suites that do not check assertion counts (namely mochitest). It also ensures that developers will notice these assertions when testing locally. Remember that any time you see one of these assertions you are seeing a potential sg:crit (via a use-after-free on an object that’s reference count is too low due to AddRef racing with another operation) and should file and fix it immediately.