Some of the heavy metals are likely from the fire retardant, and some are likely from the fire. Look at zinc vs lead for example. There is little lead in the unused sample vs the environmental samples, thus most of the lead is likely not from the fire retardant. I would guess the most likely source is lead from roofs of burning houses.

Zinc on the other hand is present in all samples in about the same amount, including the unused one. That means that the zinc is likely from the fire retardant rather than the environment. Other metals are present in slightly higher amounts in the environmental samples, and often only in some of the samples. In that case both the fire retardant and the fires/environment are likely to contribute.

To me it seems like copper, lead and manganese are mostly from the fires, while zinc and chromium seems to be from the fire retardant. Then there is the sample from the Franklin fire, that seems to be higher in everything.