From MozillaZine Knowledge Base
This preference determines what types of builds auto-updates will look for.
Possible values and their effects
(The type of build you originally downloaded determines the default value of this pref.)
All final releases.
Special release channel for extended support releases.
Betas / release candidates (up to Gecko 2.0 also final releases).
All nightly builds.
First checked in
Has an effect in