When I was a Part-time (and very gullible) Fire Officer in'62, the old hands told me that the stuff was the drainings from abattoirs.
Apparently, "pre-mix" (the mixture before it has air injection in the Foam Monitor), is an excellent dispersant when sprayed onto agitators who are threatening to climb over the airfield fences or otherwise making nuisances of themselves.
They stink to high heaven for a week, it seems, and there is no way of washing it off.
D.