firmware-brcm80211 software package provides Binary firmware for Broadcom 802.11 wireless cards, you can install in your Debian 9 (Stretch) by running the commands given below on the terminal,
$ sudo apt-get update $ sudo apt-get install firmware-brcm80211
firmware-brcm80211 is installed in your system.
Make ensure the firmware-brcm80211 package were installed using the commands given below,
$ sudo dpkg-query -l | grep firmware-brcm80211 *
You will get with firmware-brcm80211 package name, version, architecture and description in a table
| Distro | Debian 9 stretch |
| Section | Kernels |
| Package | firmware-brcm80211 |
| Version | 20160824-1 |
| Description | Binary firmware for Broadcom 802.11 wireless cards |
| Sub Section | Kernels |