Hard agree that HTTPS inspection on a firewall is bad news. It's extremely resource intense on the firewall, and stinks of "crunchy outside, squishy inside" security design to me. If your security design could also describe a delicious candy, you don't want to do that.
@SCC What features are you looking to have? The advanced security license will get you access to AMP for HTTP-based downloads, an IDS/IPS, easy l7 geoblocking, and URL-based content filtering.
If you know what you need to have, then we can help you figure out if the MX might be a fit. If you just need an "NGFW" to check off the box, then I think you need to really think more about what exactly you need and why.