there are a lot of resources online for putting a snapped board back together. www.swaylocks.com would be a good place to start.
I dont see why putting a snapped firewire back together would be any different than any other board...i mean the parabolic stringers are toast now..basically putting a board back together means connecting the halves with resin (not sun cure!) with the rocker trued up, glassing in 2 or more new short stringers, and then glassing over the whole repair area, hot coating and then sanding. If that means something to you then try the step by step instructions you can get on swaylocks or elsewhere, if not take it to a shop that has a good repair guy. Its not going to ride the same, and will be noticably heavier.
My advice is If you haven't done a lot of ding repair, and you really care about the board...then dont try this..it will be a nightmare. But since its broken, you really dont care anymore and so why not try it? 1 pint of epoxy resin, 1 yard of cloth, sandpaper and some tiny scraps of thin plywood for the stringers are all you can lose...oh and countless hours but its flat anyway.