hi krchat, i don't think it's a matter of security as both option will be secure (provided you are using a Use a Primary Password to protect stored logins and passwords in firefox). with the built-in password manager you have a closer integration into the browser whereas with keepass you have other benefits like better organisation, pw generation etc...

What about secure technology of saving passwords and usernames in Password Managers. Yes, PM is very useful tool for automatic online using it in Browser access to web resources, but is it secure for using it even with Master password.

firefox in essence uses the 3DES (DES-EDE-CBC) algorithm to secure your passwords with a master password - for more technical details about this see https://raidersec.blogspot.com/2013/06/how-browsers-store-your-passwords-and.html#firefox