There are all sorts of additives that will improve the fire performance of your foam. The biggest issue will be the processing of the material into the foam matrix without the production of nuisance and hazardous dusts. I would say you should do some reading around some of the following additives and perhaps search for more. Then trial a few. Here are some suggestions:
Antimony Trioxide. Zinc Borate. Brominated Hydrocarbons (not as hazardous as they sound these days - there are some approved ones - try the Dead Sea Bromine Group for a range of products). APP's. ATH.
Maybe a blend of more than one type - the APPs come in a range of molecular weights and phases. All of these work in a different mechanism and you should trial several IMO.
One of the things to look out for is a trade off between fire performance and mechanical properties of the foam. eg. - more FR product = better fire performance but probable less impact resistance and flexability of the foam.
Hope this helps,
P.