Actually you can, just about. I've got b2gdroid[0] installed on my Android phone, which gives you the Gaia experience as your home screen, and access to the built-in applications as well as being a launcher for your Android apps.

It's not ideal, as the integration points don't appear to be fully worked out and it doesn't always perform very well, but I'm sure it could be turned into essentially a downloadable FirefoxOS as a hook to try to get people to get a native device next time, if some development effort was available.

[0]: https://github.com/fabricedesre/b2gdroid