As a guy who works in the ARFF business, here is my 2 cents worth..
The foam used by ARFF crews these days is called AFFF (Aqueous Film
Forming Foam) and is used to suppress vapors from fuel (it's the vapor that
actually burns). That is what the stuff (AFFF) was made for. It is mixed at a 3
or 6% mixture with water, which means that it does not hang around for long
on pavement, It would run off the runway much as rain water would, therefore
not providing any protection. Remember also that those big "crash trucks" only
carry so much agent, agent that might be needed for fire suppression and
rescue activities after the a/c arrives. If you use up all of the AFFF before
the guy arrives, and then need the stuff, you'll have a big problem on your
hands..
I have not heard of "foaming" a runway being done in the United States
for decades. Just is not done anymore..
Tom