I have an ISA Server setup as a web proxy in our domain environment . I have changed the proxy settings to the address of the new web proxy server for all the users in the domain by using group policy . Even the connection tab on the internet options have been disabled so that users wont be able to view the settings thereby preventing them from changing the proxy settings . It has been completed successfully . But when we tested the settings with various browsers by trying to access the blocked sites in our environment such as Facebook and other social networking sites , IE , Safari and Chrome seems to be detecting the proxy in the network and was blocking it but Mozilla Firefox and Opera does not seem to be affected by the change and users could have unrestricted access to the web . Please help me with how to configure mozilla firefox proxy settings using group policy edit .