Firefox 90.0.2 on Windows 10 was installed on 7/27/2021 which is about when this started. Firefox is suddenly asking for master password twice, while running, not at startup. I leave Firefox running at night. I find two password dialogs when I log in in the morning. I'm not sure if they display at a certain time (e.g. midnight), or if it is a result of the system going into sleep and waking. When Firefox starts or restarts, it displays one master password as expected. I found [https://support.mozilla.org/gl/questions/1262314 this article from two years ago] , but the "hotfix-update-xpi-signing-intermediate-bug" is not found in about:debug or in profiles. I have not installed any add-ons recently. Last Windows update was KB5004237 installed on 7/18. Is it Firefox? What else could be causing this problem?