Several things, but ultimately I had been at Mozilla for 6 years so that's long enough to start to get an itch for something very different. I'm interested in a lot of things so I was getting annoyed that I didn't have time to pursue them (some of the programming-related, some not).
The team I was on was going through a lot and I got pretty burned out about all the various politics, changes, and dynamics with most of Mozilla. As said in another answer, I don't think Mozilla is bad place and most medium-sized companies are faced with these issues. But the idea of going out on my own was really appealing, with the ability the focus mostly on things I wanted to do.
I'm also trying to figure out the rest of my career generally, so this is a good time for me to work on a lot of different things and try to figure out where I want to focus on next.