From MozillaZine Knowledge Base
This preference determines whether to use of the operating system's NTLM implementation to silently authenticate the user with their Windows domain logon with proxy servers.
Possible values and their effects
Automatically authenticate with proxy servers. (Default)
Prompt for authentication.
First checked in
Has an effect in
- Netscape (all versions since 8.0)
- Mozilla Suite (all versions since 1.7.3)
- Firefox (all versions since 1.0)
- SeaMonkey (all versions)