Well, hmmm, it would have been better for you to not say you no longer worked there. Probably going to have to address that somehow. If your boss fired you, how are still working there? If you are on notice and defiantly in the final week/s of employment then it's not too hard to justify saying you no longer work there.
As to what to say, probably something along the lines of there was a serious personalty conflict and just take the ding that is going to imply. I don't think there is a sure fire clean way out of it, so might as well be upfront and concise. As such you may also want to be honest about the reason for leaving, or at least fold it into your reason for leaving. "The depts where merging, which along with some personality conflicts made it more attractive to move on..." or some such.

As a boss I am going to be a lot more leery of someone who is obviously trying to hide something over someone who can admit previous problems. Both are things to watch out for, but one is a lot worse than the other.