If you were missing firmware, that’s not actually a driver issue. You do need the firmware and (unless you also installed the professional drivers as well) you should be all good now and using the full open source stack.
Anyway, glad to hear it’s working for you!
Do you mean it constantly does it when a monitor is turned off or that when you initially turn off a monitor, it rearranges all windows to fit on the remaining monitor.
If the first, I’m not sure what the problem might be, but the second is pretty normal, I think. The card sees that the display was detached and moves your windows to the attached display so you can see them.