When i contacted their support they just said there was no known problems on the current firmware which doesn't make sense based on the changes they have made since then.
The last english firmware was 6.20 in october last year. The german firmware has had 6.23 and 6.30 since then.
Changes include everything from security to vdsl interop changes to wireless stability improvements.
Google translate if you can't read german:
http://ftp.avm.de/fritz.box/fritzbox.fon_wlan_7390/firmware/deutsch/info.txt
