I tried to install it on firefox 50.0 with avast 12.3.2280 on my win7 system as I didn't use it on that, only on this test system.
It failed to install (disabled) and it is the same AOS version 12.0.88 as in my image2 in Reply #7 above.
Despite having the same about:config settings for xpinstall.values in my first image in Reply #7 above.