The binary firmwares are downloaded from http://ftp.debian.org/debian/pool/non-free/f/firmware-nonfree.

This file contains firmware data derived from proprietary unpublished
source code, Copyright (c) 2004, 2005, 2006, 2007 Broadcom Corporation.

Permission is hereby granted for the distribution of this firmware data
in hexadecimal or equivalent format, provided this copyright notice is
accompanying it.