WTF is nightly and how did it get installed on my system?

OK. I know it's from mozilla but how and why is it on my system? I've just done a windows update and there's Nightly. Recently installed and recommended.

I love firefox, mainly because it's not chrome. (Don't really have anything against google except for the fact that they are too big and powerful and need to die.)

Browsers are so integral to what's going on on the desktop that I cannot believe that I installed this. Especially since I've never believed that private browsing is in anyway useful or private.

What happened?