Of course, this helps you very little when the malware is running under your user account and uses DPAPI calls to decrypt the passwords.
But none of that is going to help against an attacker with the same permissions.
But as you pointed out, another process with the same privileges can decrypt it making it pretty pointless in both cases. Only way to securely do it is to prompt the user for a decryption key each time they open the browser which has usability issues but Firefox offers it via the Master Password functionality.
[0] https://msdn.microsoft.com/en-us/library/windows/desktop/aa3...
On macOS, applications address spaces are isolated and code signing certificates are used for identifying application requests to the keychain.