The easiest way to get Bluetooth is to install all the proprietary Atheros firmware with these commands:
wget http://ftp.us.debian.org/debian/pool/non-free/f/firmware-nonfree/firmware-atheros_20190114-2_all.deb
sudo dpkg -i firmware-atheros_20190114-2_all.deb
If you want to only install the proprietary firmware for the Atheros Bluetooth, you need to do it this way:
sudo apt install git
git clone https://git.kernel.org/pub/scm/linux/kernel/git/firmware/linux-firmware.git
(Wait while it downloads 360MB)
cd linux-firmware
sudo cp -r ar3k /lib/firmware/
If you want to delete the 360MB that you downloaded:
cd ..
rm -r linux-firmware
@nicole.faerber, @mladen, @joao.azevedo,
In my opinion, these instructions should be added to https://docs.puri.sm