What security software do you have?

Some security software uses this to inject their root certificate in Firefox to be able to intercept internet traffic.

• https://support.mozilla.org/en-US/kb/error-codes-secure-websites

You can check the about:policies#active page to see whether policies are active (63+).

• https://support.mozilla.org/en-US/kb/see-active-policies-firefox-enterprise

You can inspect the Mozilla and Firefox keys with the Windows Registry Editor in HKEY_LOCAL_MACHINE and in HKEY_CURRENT_USER with the Windows Registry Editor to see whether GPO policy rules are active.

• HKEY_LOCAL_MACHINE\SOFTWARE\Policies\Mozilla\Firefox\

Note that the mere presence of the "Mozilla\Firefox\" key is sufficient to make Firefox display this notification, so if you have the Firefox key then remove it and only leave the Mozilla key or remove this key as well if it is empty.

• HKEY_LOCAL_MACHINE\SOFTWARE\Policies\Mozilla\Firefox\ =>
  HKEY_LOCAL_MACHINE\SOFTWARE\Policies\Mozilla\

That's pretty common. It usually comes from your antivirus program, since it's essential to allow these programs to scan your internet traffic. It can be removed, but it will usually break functionality in your antivirus programs, so it's not recommended.