From MozillaZine Knowledge Base
On Windows NT, 2000, and XP, when a dial-up connection is configured and a certain site cannot be reached through active connections by a Microsoft application, Windows will automatically attempt to use the dial-up connection to reach it. Non-Microsoft applications must use the RAS service to have the some functionality. This preference determines whether to use the RAS service.
Possible values and their effects
Use the RAS service to attempt to autodial. (Default on Windows NT, 2000, and XP)
Don't attempt to autodial. (Default on other OSes)
First checked in
Has an effect in