See:

• https://support.mozilla.org/en-US/kb/password-manager-remember-delete-edit-logins

Can you manually create a new login?

• click the '+' at the top of the about:logins page
• https://support.mozilla.org/en-US/kb/add-login-firefox-password-manager

Yes, I can manually create a new password. I, in fact, had to do that. What happened to the popup?

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