It's issued by Firefox, and seems legit. Give it a try. I was very concerned myself about this option. Now, I believe we can get the JoS website in it's current form. Which is awesome.
I was riddled with how to get a PDF made for the past few days, I knew it would take a whiiile, and I would need permission from High Priestess Pythia.
So, I found this much quicker get around. Give it a try.