Overall, from personal experience, I felt that Palm had the best developers' support. They really tried hard to help us. It didn't work but they tried. I attended a Firefox OS App Day at Mozilla's office in Toronto and was completely disappointed. They just don't understand developer support. If they go at it the way they did yesterday, I predict they'll have an even harder time than Palm who actually put in some real effort to attract developers.
Wrote a lot more about my crappy experience here - Firefox OS Is Not for Serious Developers