The following applies to Mozilla Firefox ESR 52.9.0 only!
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Open "about:config", and then:
Add new preference: media.gmp-eme-adobe.enabled (boolean; true)
Add new preference: media.gmp-eme-adobe.forceSupported (boolean; true)
Add new preference: media.gmp-eme-adobe.visible (boolean; true)
Add new preference: media.gmp-eme-adobe.version (string; 17)
Change preference: media.gmp.decoder.enabled (boolean; true)

Open "about:support", and then:
Profile Folder > Open Folder
Inside the profile folder, create subfolder "gmp-eme-adobe" and open it
Inside "gmp-eme-adobe", create subfolder "17" and open it
Extract the provided ZIP archive into folder "17"
Restart Mozilla Firefox

Note: Some Windows XP x64 users may get BSOD C2 error after installation.
To solve this issue, install KB3070102