Since you don't mind the weather you will have a great time. I've gone 3 times the end of January and this year the beginning of February. The weather has been nice almost every day. One day this last time it was very chilly, but the other days, including all the other trips, it was nice. I would start the day out in shorts, short sleeve shirt and a sweatshirt and before noon I would have shed the sweatshirt. I'd have it with me in case I needed it before park closing. We've also have VERY little rain.
I didn't watch the MK fireworks, but I know that they were offered at least a time or two during the week.
I agree with other posters that I like the shortened hours. Since it isn't crowded we do everything that we want to do, but are not "burning the candle at both ends". It's amazing how exhausted we still were every evening, though.
This last trip I went with my sister and my 4yo, I will go with just my 4yo this October, and hopefully my 4yo, my sister, and my 81yo mother this December. Then I have to figure out how to squeeze one more trip in maybe the end of January before my AP expires...
T&B