This does not happen anymore after a browser restart, suggesting that whatever state that has caused this was entered after some extended use of the browser.
Bug 1692343 Comment 1 Edit History
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This does not happen anymore after a full browser restart, suggesting that whatever state that has caused this was entered after some extended use of the browser. However it did continue happening after the browser was restarted via the `File > Restart (Developer)` menu. In the latter case I believe the parent-most firefox process is not actually replaced, or restarted.