Firefox doesn't trust servers with your Windows password out of the box; you need to add each server to a list using the about:config preferences editor. (1) In a new tab, type or paste '''about:config''' in the address bar and press Enter. Click the button promising to be careful. (2) In the filter box, type or paste '''ntlm''' and pause while the list is filtered (3) Right-click the "network.automatic-ntlm-auth.trusted-uris" line and choose Modify from the pop-up context menu. (If you have changed this setting before, the line will be bold.). (4) In the dialog that appears, type the name of the server you want to approve for automatic sign-in and click the OK button. (If you need to list multiple servers, separate them with a comma and no spaces.) You only need the host name, not any of the folder or page parts. You're done in about:config. Try your internal site again. Any luck?