Hey there,
this is not true at all, at least in my case. You can deactivate NAT in the IPFire and put some routing-rules in the Fritz!Box (I own a 6591 cable). Works like a charm and no double-nat anywhere. 
Greetings
Alex
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If the router does the NAT, some functionalities of IPFire depending on this network separation may not work as expected, IMO.
Hey Bernhard,
I’d really like to know which functionalities could/would not work in such a setup, do you have any examples? And please with explanation why, this could save a lot of time when problems may occur.
Greetings
Alex
NAT does a separation of two networks. On this transition a firewall may block packets. This one of the main functionalities of IPFire: packets from WAN ( red if ) are allowed only, if they belong to a connection initiated from the LAN ( green and blue if ).
If the direct internet access device is a router, these rules can be set by the owner. If this owner is the ISP, a filtering can be established.
If the device is just a modem ( bridge mode is equivalent, IMO ), the traffic arrives unfiltered the IPFire device.
Another topic is the availability. With a modem the availabity of the access medium is mapped to the ethernet port ( the logical internet access medium of IPFire ).