Bug 867755 introduced the use of SupportsWeakPtr in imgStatusTracker. Unfortunately, as mentioned in that bug, the interface of SupportsWeakPtr is unsafe for multithreaded use (and questionable in general). imgStatusTracker should use a safe weak pointer implementation that involves acquiring a strong pointer to the object pointed to by the weak pointer, rather than just checking whether the weak pointer is null once and hoping that result stays valid for the rest of the function. Sadly, such an API does not exist at the time of this writing, so step one is probably to fix SupportsWeakPtr or create a new one.
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