It's a conceptual question:
Suppose i want to separate
firewall, can it help to my security?
INTERNET >>>> NAT >>>> Firewall >>> MyNetwork
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I don't think it would make much of a difference. NAT has nothing to do with security and the firewall should be your primary defense.
Having a device provide both firewall and NAT services isn't a security risk by itself. The only potential security risk would be the software and configuration of the device. Since there are so many vendors/products on the market and questions about products is disallowed, this cannot be answered anyways.