Master password protects your stored logins and passwords. Without it, anyone can see and use the passwords you store in your firefox browser. So if you are saving passwords to login to any sites (especially for emails and banking) for convenience, a master password can make it secure.
Take note that the security of your saved passwords depends on the strength of your master password and whether you share it to any body or not. So it is important that your pick a password that is hard to guess and that you do not share it or if you do, you share it to the person who you really trust.
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