Are you using Sync?

Start Firefox in Troubleshoot Mode to check if one of the extensions ("3-bar" menu button or Tools -> Add-ons -> Extensions) or if hardware acceleration or if userChrome.css/userContent.css is causing the problem.

• switch to the Default System theme: "3-bar" menu button or Tools -> Add-ons -> Themes
• do NOT click "Refresh Firefox" in the Troubleshoot Mode start window

• https://support.mozilla.org/en-US/kb/diagnose-firefox-issues-using-troubleshoot-mode
• https://support.mozilla.org/en-US/kb/troubleshoot-extensions-themes-to-fix-problems

Thanks for the suggestion, cor-el. I haven't had time to try Troubleshoot Mode yet.

I am using Sync.

Strangely, since I posted this message, the problem has become less frequent. Perhaps this is because I did Help-> More troubleshooting -> Clear startup cache.

I think I've discovered a pattern in what provokes this problem: I clear site cookies and then refresh a page (ctrl-F5), and shortly after (not necessarily immediately) I get the prompt.

Are you canceling the PP prompt because that will (re)lock the passwords and prevent access?

You can temporarily disable the Password Manager by removing the checkmark on "Ask to save Logins..." to see if that has effect. You can still use the right-click context menu to fill a login in this case, assuming that you have entered the PP.

You can inspect the current login state and login and logout and change the Primary Password via the Security Device Manager.

• Settings -> Privacy & Security
  Security -> Certificates -> Security Devices

In the left panel select "Software Security Device". You can find "Log In" and "Log Out" and "Change Password" in the very right panel. You may have to Zoom out the page or go full screen to make this right panel visible.

I see that after I "Clear cookies and site data" from one site, the Software Security Device changes from "Logged In" to "Not Logged In".

That may be the issue, I have the same problem posted a long time ago. I also clear cookies and site data by right click on padlock for a certain site I know leave a lot of junk. Maybe that is causing the prompt for password to come up again- even though the site I clear doesn't use a password logon, I just view.

I did what Co-rel said and now it shows logged in as the only option is to log out, change password and load. Will check it next time it wants password.

That is it, I tested it out even though I just view that site, it has no passwords. If I clear cookies and site data by right click on padlock, it then shows I logged out when looking at certificate security devices. Strange, wonder if this is error or why it does it when that site isn't logged into. I should correct, I could log into it, but rarely do. And for this test I did not log into it.

In my case, the site in question does have a login, although I don't log in to that site (and have no password for it), and there's no visible login box either before or after I clear cache and refresh (although there is a login button; and after the refresh, there's a prompt to login using my Google Account).

It seems that something about clearing the cache logs me out from the Software Security Device, which doesn't seem like correct behavior.

Agree it shouldn't be doing this, I guess we are the only ones in the whole wide world that right click on padlock to clear cookie and site data and have a primary password we use.  :)