Thanks Ben
I was using the Enterprise Toolkit docs which state to use the MSI Property, not the reg key.
So if the MSI Property is no longer working for 11.x and higher, perhaps Adobe should actually update their web-based version of the customization kit to state this?
I've queried the Mozilla folks for how to programatically disable the plugin (I'm sure it can be done, I'm just not sure how).
I know in FF ESR 17 they have a way to disable the built-in PDF viewer.