I think it may be cause by different widget hierarchy. We hide tooltips on any event we get (keyboard/mouse) and that's handled by FF itself at nsXULTooltipListener::HideTooltip().
Bug 1798131 Comment 10 Edit History
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It may be caused by different widget hierarchy. We hide tooltips on any event we get (keyboard/mouse) and that's handled by FF itself at nsXULTooltipListener::HideTooltip().