There is one pretty big difference. Android is developed by google behind closed doors and then released to the world once they think it is ready. You don't get to see the code or influence it during development. Firefox OS is developed in the open, all the code is on github and you can join the lists and participate on the process.

The development process is arguably more open - everything happens in public repositories, many different individuals and companies are involved at all stages, etc. Android does parts of the development in a private tree, see http://source.android.com/faqs.html

Android itself is definitely open-source in a technical sense though, and they share a ton of useful changes back to the kernel and userspace apps that Firefox OS and many other projects use and contribute to as well.