FireDelayedEvent, while declared in DocAccessible.h, is only defined in DocAccessible-inl.h, and not all .cpp files using the function include the header.
Bug 1833432 Comment 2 Edit History
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FireDelayedEvent, while declared in DocAccessible.h, is only defined in DocAccessible-inl.h, and not all .cpp files using the function include the header. That it builds in the normal build configuration is kind of an accident. (It's worth noting that I can't reproduce.)