Hi,
I have the 2310 but have not bothered with a firmware update and to be honest i can't see how that is possible...the amp has no USB socket or even a network connection so how you go about getting the firmware onto the amp is anyones guess!? I presume this is a service Denon would have to do for you meaning sending the amp away? Unless you are experiencing problems with it i don't see the need for an update unless it adds content which is extremely unlikely! Hope that helps.
EDIT: This is from the Denon uk website and confirms what i said above: "DENON recommends you to update your unit only in case where some problem has been encountered.To update the firmware of your product feel free to contact the authorized DENON-retailer where you have purchased your DENON product. Phone number and the address of your retailer you can find on here."
This means the amp will need to be sent away for the update to be applied by your retailer and as they state, it's not advisable unless you have encountered a problem so i wouldn't even consider it....especially when you see the warning that Denon accepts no responsibility for any problems caused by the update. Forget all about it and just sit back and enjoy your receiver