https://www.adobe.com/

https://get.adobe.com/flashplayer/

Flash Test Page

https://www.adobe.com/software/flash/about/

Firefox

install -d ~/.mozilla/plugins/ &&
install -m644 libflashplayer.so ~/.mozilla/plugins/

DRM

The only Adobe Flash player version, supporting DRM is the one distributed in Chrome OS. 

strings -a libpepflashplayer.so | grep PPB_Flash_DRM

PPB_Flash_DRM;1.1
PPB_Flash_DRM;1.0

With the help of the freshplayerplugin project, libpepflashplayer.so can be used together with Firefox.

Get libpepflashplayer.so from Chrome OS
export CHROME_OS_RECOVERY_IMAGE_VERSION=9901.77.0 # Flash Version 27.0.0.187
wget https://dl.google.com/dl/edgedl/chromeos/recovery/chromeos_${CHROME_OS_RECOVERY_IMAGE_VERSION}_zako_recovery_stable-channel_mp-v3.bin.zip
unzip chromeos_${CHROME_OS_RECOVERY_IMAGE_VERSION}_zako_recovery_stable-channel_mp-v3.bin.zip
sudo losetup -f --show -P chromeos_${CHROME_OS_RECOVERY_IMAGE_VERSION}_zako_recovery_stable-channel_mp-v3.bin
sudo mount -o ro /dev/loop0p3 /mnt/1
sudo install -m644 /mnt/1/opt/google/chrome/pepper/libpepflashplayer.so /usr/lib/

The URL of the recovery image is taken from the recovery.conf file, following this URL chain:

Install and configure freshplayerplugin
git clone https://github.com/i-rinat/freshplayerplugin.git
mkdir freshplayerplugin/build
cd freshplayerplugin/build
cmake ..
make -j $(nproc)
install -d ~/.mozilla/plugins/
install -m644 libfreshwrapper-flashplayer.so ~/.mozilla/plugins/

~/.config/freshwrapper.conf: 

pepperflash_path = "/usr/lib/libpepflashplayer.so"
enable_hwdec = 1
Test