Hi there, thanks to all for all this gathered information! I'm using FireFox 52.5.0 ESR on Windows XP x64 Edition SP2 and having an odd issue which I didn't see others having while scanning through the thread.
I can install the plugin by modifying the about:config as specified and dropping the plugin in the profile folder, going to youtube.com/html5 reveals that it's working, but then when I close and re-open FireFox, the plugin is removed! I'll close FireFox, re-open it, and as FireFox opens the 17 folder is deleted from within the gmp-eme-adobe folder, and many of the flags set in about:config are completely gone.
I'm wondering if this has anything to do with being signed in to my FireFox account, so I'll probably wipe FireFox clean and try again without signing in and let you know how it goes. Thanks for reading!
I found out what was causing my issue. For Windows XP Pro x64, the media.gmp-eme-adobe.abi string must be set to x86-msvc-x64 as opposed to x86-msvc-x86, otherwise the plugin is removed when FireFox is relaunched and the version, abi, and lastUpdate lines are all removed from the config. If another human being exists in the world that uses Windows XP x64, hopefully that's useful