firmware-atheros software package provides Binary firmware for Atheros 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-atheros
firmware-atheros is installed in your system.
Make ensure the firmware-atheros package were installed using the commands given below,
$ sudo dpkg-query -l | grep firmware-atheros *
You will get with firmware-atheros package name, version, architecture and description in a table
| Distro | Debian 9 stretch |
| Section | Kernels |
| Package | firmware-atheros |
| Version | 20160824-1 |
| Description | Binary firmware for Atheros wireless cards |
| Sub Section | Kernels |