[balac]

He is likely referring to: https://www.mozilla.org/en-US/firefox/developer/?v=a

This is a developer focused edition of FF and shouldn't be unstable.

[Manishearth]

Dev edition is Aurora, which is basically alpha (more unstable than beta).

So you get new features more rapidly, and a lot of the feature flags are flipped on, without needing to use something as unstable as nightly.