By default, Internet Explorer is the only browser that can pass your Windows login credentials onto web sites. In fact, Firefox doesn’t even give you the option of remembering your login details; you have to re-enter them every session.

However, there are a few tweaks that can be made to make both IE and Firefox use your Windows login credentials for certain sites:

1. Open up regedit.
2. Locate registry key HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\Lsa.
3. Add a new DWORD Value called DisableLoopbackCheck and set the value to 1.

Then in Firefox:

1. Browse to about:config.
2. Enter uris into the filter box.
3. Enter a comma separated list of you sites into the three preferences* that are shown. E.g. local-1,local-2

If IE still/now shows a prompt after making these changes:

1. Go to Tools > Options.
2. Select the Security tab.
3. Select Local intranet.
4. Click Sites.
5. Uncheck the ‘Automatically detect intranet network’ setting.

* The three Firefox prefs should be ‘network.automatic-ntlm-auth.trusted-uris’, ‘network.negotiate-auth.delegation-uris’ and ‘network.negotiate-auth.trusted-uris’.