A keylogger on your system could keylog your master password. A password manager itself has a similar threatmodel but it holds (usually) more valuable informat

A keylogger on your system could keylog your master password. A password manager itself has a similar threatmodel but it holds (usually) more valuable information than just passwords (in my case TOTP, Bank Data, Credit Card, etc.) PM's are also usually better equipped to deal with this (Keepass uses a Secure Desktop on Windows) Firefox's master password is "good enough", so to speak. It's unlikely someone will attack you either way but if someone does then FF's system should hold out a day or two if you picked a reasonable password (a modern PM can put up much better brute force defence)