The real story. The so loved 'foam' contains a thing which is what actually cleans up the fire, called CO2. So why not use pure CO2 or BUBW in powder toy? Make some researching before suggesting kay?
Foam was actually in tpt a long time ago, it then got replaced by INSL. Anyway, as said, if all you want is too kill fire, there are already elements that can do that...
I'd say bubbly water is the closest element to "foam" because it contains CO2 bubbles in it, and foam IRL has bubbles in it.