If you want to eat on new years eve its better if you book somewhere, plus you may have to leave a deposit to secure your booking. Fireworks, you need to be in the town square at midnight, last year was good, but loud. At midnight and after, the bands in the mehenas from around the square come out and play in the street for a bit, much fun to be had, plus the fireworks keep on going.
There are some mehaneas with websites, but not many. check out http://bansko.bg/index_en.html and then click on pubs in bansko at the bottom. Theres a few on there that are good. If not when you get out there try and go into one a few days before and book a table. http://bansko.bg/index_en.html