I second the previous poster who says discounts are extremely unlikely, however keep checking you never know. We saw in the Millennium at Epcot, that was a New Years party.
Expect huge crowds, it will be the busiest you will ever see the parks. Book your restaurant/s & fast passes & get there reasonably early, although guests are the last group to be excluded during phased closures (which will almost certainly happen at some point) it probably isn't something you want to risk.
Expect insane queues for all rides & long long queues at all the qs locations. Pack a sense of humour temper your expectations & you will have the most amazing birthday.
Oh & one crucial piece of advice, getting into toilets takes time, a long time, particulary for the ladies so take the opportunity to use restaurant ones when you are there as they will be the cleanest & quietest. Finally once you are in your chosen firework viewing location, try not to move because getting back to your group will be very difficult, the later it gets the harder it will be.
Have a blast.
