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.
Re:Firebug lives on in Firefox DevTools
by Manishearth on Tuesday December 20, 2016 @12:59PM
(#13222212)
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.
I wonder if that meant the latest developer release, which would be expected to be unstable?