although both prop firms and forex brokers have demo accounts, succeeding on a forex broker demo account does not mean you will succeed on a prop firm demo account because prop firms come with alot of rules designed to make it hard to succeed.
For example prop firms have daily drawdown and maximum daily loss limits, some prop firms dont permit trading during news releases, some prop firms dont permit using expert advisors, some dont even allow you engage in high risk trading etc. Breaking any of these prop firm rules will lead to them cancelling your account and you will need to pay for a new account.
So, although a trader may make profits on a demo account, he needs to practice on a live account to be able to develop control over his emotions which he cannot do on a demo account.