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, and is found in channel-prefs.js of the application directory. Software_Update#Update_channels_-_Advanced has more detailed description of these channels.
All final releases.
Special release channel for extended support releases.
Betas / release candidates (up to Gecko 2.0 also final releases).
All nightly builds.
No updates are offered.
First checked in
Has an effect in