It works for me on Firefox 50.1.0
But the move away from XUL is indeed worrisome for plugins like these...
I kinda hope someone will just build it into the servo project and make that into a usable minimal browser.
I'm a little wary of installing addons from some random github repo.
FWIW, I use willsALMANJ's releases with no problems, he was/is quite active on the pentadactyl issue tracker.