I am pretty sure it stays closed. My friend used to be a cast member and she told me this story about how she and another CM once were in a transport vehicle in Toontown attempting to vacate the area before the fireworks started. They didn't make it in time, so had to "shelter in place" in the vehicle until cleared to leave. She described shells falling all around them... sounds crazy! I imagine it takes a long time to clean up and inspect the area after the fireworks. This was many years ago so I don't know if the protocol is still the same.
That is what I thought so I was really confused when my ridemax plan had me visiting toontown in the hour before close. I just realized though that there are no fireworks that night lol. So on night's without fireworks it would be open until park close?