Let's not exaggerate and scare people. If you're in the center of the hub at fireworks time, yes, it's packed (you can still move, it just requires a lot of "excuse me"). However, beyond that, it's not crowded to the point you are unable to move.
MamaBear12, don't get too nervous about it. Yes, it's the busiest week of the year. No, you won't be able to do as much as you do during the summer...lines are longer, 4th, 5th FP+ are harder to get (but not impossible by any stretch). However, as I said before, if you have the right attitude and have some patience, you'll have a wonderful time. Lots of people in the holiday spirit, lots of neat Xmas decorations. Go with the flow, take your time and enjoy. We're many, many time summer veterans and had the same concerns as you when we did our first Xmas at Disney in '15. We had a fantastic time, especially once we came to grips with the fact that we weren't going to ride as many things as we normally do and just changed focus a bit. We're heading down tonight and staying through New Years. Gonna be awesome!!!