I apologize if this has been answered, but I've been wanting to play my legit copy of BF3, but I'm afraid to update to the latest firmware that came out on Dec 6. Can I update, or will that mess up my modded firmware? Thanks for any help.

It depends on what version dashboard you're currently on.
If you're on 2.0.13599.0 or above you'll be fine.

If you want more info look up 'Xbox 360 system software' on Wikipedia.

Well, I was on LT 2.0, and whatever was the official firmware before this one. I stopped going on Live after they did the silent update.

You're fine. Have fun. Just don't try playing ap2.5/xgd3 burned games for the moment until LT+3.0.

Thanks!