Follow these instructions if you are using the b43 driver from linux-2.6.25 or compat-wireless-2.6, or from any current GIT tree.
b43-fwcutter version 011: available in latest Linux distributions.
Broadcom's proprietary driver version 126.96.36.199: download at: http://mirror2.openwrt.org/sources/broadcom-wl-188.8.131.52.tar.bz2
tar xjf broadcom-wl-184.108.40.206.tar.bz2
b43-fwcutter -w /lib/firmware wl_apsta_mimo.o
Note that you must adjust the FIRMWARE_INSTALL_DIR path to your distribution. The standard place where firmware is installed to is /lib/firmware. However some distributions put firmware in a different place.