And I imagine that those anti-virus vendors also have their own Password Manager for Firefox either as part of their deluxe version or as a separate application.

The Firefox Logins Manager is safe, plus for additional "protection" you can enable the Master Password feature in Firefox. https://support.mozilla.org/en-US/kb/use-master-password-protect-stored-logins

The Master Password is good for situations where other people may use your PC in your own Windows Logon User Account (LUA), if you don't have a "guest" LUA for them to use, which is highly advised for "guest" use or when more than one person regularly uses than same PC to keep other from being able to access your own data (and you from accessing their data).

You should always use a master password to protect the passwords. Without a master password u only need to have access to the logins.json and key4.db files to be able to access the passwords, so if you would get malware then your passwords can easily be compromised. The malware could be programmed to remove itself after having send this information to a server and you may never notice this.

Thanks for the responses.