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Looks like the following will stick around until at least the regular version of Firefox 53 and will remain in Firefox ESR 52. Confirmed that this is currently working for FireIE in regular FireFox 52.

about:config
add new boolean: plugin.load_flash_only, set to false

http://www.omgubuntu.co.uk/2017/03/force-enable-firefox-52-npapi-support

https://bugzilla.mozilla.org/show_bug.cgi?id=1269807