The kernel version largely dictates which firmware is being used although the
carl9170 firmware was available for earlier releases too. Now only the
carl9170 firmware is compatible as the older ar9170 driver was removed (which
used the older ar9170 firmware). There is an even newer carl9170 firmware for
3.1+ kernels. The 5.0 release has the older ar9170 firmware and I think 4.01
and 4.5 also have it (it was added after the initial release though of these
versions).
Debian has the ar9170 driver and carl9170 drivers depending on the kernel you
install. I think the firmware has to be installed separately still. I forget
if it is packaged or not. The installation is easy though. You just have to
download the firmware to the /lib/firmware directory. The thing with debian
is there are a lot of variations and many don't include essential utilities
for desktop use with base installations. Most derivative distributions (at
least based on Ubuntu) have the carl9170 or ar9170 driver and firmware.
Ubuntu has the older ar9170 driver and firmware in the older releases. It has
the newer carl9170 driver and firmware in the newer releases.
