From MozillaZine Knowledge Base
This is an enumerated preference: all preferences whose names start with browser.safebrowsing.provider. are read to generate the list of Safe Browsing providers. The * in the preference name is a non-negative integer, and matches the number in the other preferences needed to describe a provider. It also determines the order in which the providers appear in the options dialog.
Possible values and their effects
By default, the following preference/value pair exists:
First checked in
Has an effect in