Reproduced the issue on affecte Nightly 70 (2019-07-30) on Windows 10 x64. This is verified- fixed on the latest Nightly 71.0a1 (2019-09-09) (64-bit) and Beta 70.0b4 (64-bit). The "show password" option will be available in the doorhanger and will also disappear after dismissing the doorhanger as per expected scenario mentioned in Bug 1178855. However, please note that the specific scenario described in Bug 1576199 still affects the correct functionality of this option, marking it as blocking.
Bug 1569989 Comment 7 Edit History
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Reproduced the issue on affecte Nightly 70 (2019-07-30) on Windows 10 x64. This is verified- fixed on the latest Nightly 71.0a1 (2019-09-09) (64-bit) and Beta 70.0b4 (64-bit). The "show password" option will be available in the doorhanger and will also disappear after dismissing the doorhanger as per expected scenario mentioned in Bug 1178855. However, please note that the specific scenario described in Bug 1576199 still affects the correct functionality of this option (show password is available two times in the dismissed door-hanger - when the scenario is used).