We recently added the following item to the CertAlerts page on the FAA website:

Certalert 21-05, Part 139 Extinguishing Agent Requirements
This CertAlert informs Part 139 airport operators about changes to the military specification (MIL-PRF-24385F(SH)) for firefighting foam referenced in Chapter 6 of AC 150/5210-6D. While the performance standard remains the same, the military specification no longer requires the use of fluorinated chemicals. One acceptable means of satisfying 14 CFR Part 139 requirements is to continue to use the existing approved foam which does contain fluorinated chemicals. However, FAA encourages certificate holders that have identified a different foam that meets the performance standard to seek approval for such foam from the FAA. FAA staff are available to provide assistance or answer questions about how to pursue FAA approval for a foam that meets the performance standard but does not contain fluorinated chemicals.