AbstractThread was introduced in order to determine a generic threading model that guarantees that events always run serially; and this was done before nsISerialEventTarget existed.

Today AbstractThread is mostly used to access the direct task dispatch semantic; which gives a similar (but not identical) mechanism to JS's microtasks.

DirectTasks (and maybe StateChange tasks) should be merged into nsISerialEventTarget.

Direct Tasks should be split from the current Tail Dispatching mechanism as that was only an implementation detail and while similar aren't actually dependent on one another (see Bobby description in bug https://bugzilla.mozilla.org/show_bug.cgi?id=1631304#c0)