The new firmware is build 2C54 and replaces the previously used firmware on both AirPods and the AirPods Pro. The update brings both models together under the same build number for the first time, instead of having separate builds for each.
This marks the second firmware release for the AirPods Pro. The first firmware update, build number 2B588, was released on November 14 and only applied to the AirPods Pro, not the AirPods.
Apple does not provide release notes for its accessory firmware updates, making it difficult to determine exactly what the firmware does.