Hi Markus, webRequest events may be dispatched with a slight delay to avoid sending too many multiple small messages over IPC, the IdleDispatch is capped at 250ms and that seems suspiciously near to the 260ms that are mentioned in this bug and so think it is likely the underlying reason for the behavior described in this issue (I'm adding Bug 1587058 as a see also, because it is an issue related to re-evaluate this "webRequest event coelescing").
Bug 1684703 Comment 15 Edit History
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Hi Markus, webRequest events may be dispatched with a slight delay to avoid sending too many multiple small messages over IPC, the IdleDispatch is capped at 250ms and that seems suspiciously near to the 260ms that is explicitly mentioned in this bug and so I think it is likely the underlying reason for the behavior described in this issue (I'm adding Bug 1587058 as a see also, because it is an issue related to re-evaluate this "webRequest event coelescing").