What is the red foam the planes drop on wildfires made of?

Answer 1

It is not foam. According to the CDF Aviation wikipedia page:

"The retardant used to slow or retard the spread of a fire is a slurry mix consisting of a chemical salt compound, water, clay or a gum-thickening agent, and a coloring agent."

Many years ago the mix contained "borates" that had a tendency to be worse than simply letting the fire burn.

Under "Aerial firefighting": "Borate salts were used in the past to fight wildfires but were found to sterilize the soil, were toxic to animals, and are now prohibited. Newer retardants use ammonium sulfate or ammonium polyphosphate with attapulgite clay thickener or diammonium phosphate with a guar gum derivative thickener." [footnotes omitted]

Answer 2

The red foam dropped on wildfires by planes is called fire retardant. It is typically made of a mixture of water, fertilizer, and a red coloring agent. The purpose of this foam is to help slow down the spread of the fire by creating a barrier that prevents combustion.