How do I supply proxy authentication credentials in windows 7 or firefox 3.6?

Solution 1:

One thing to try is:

Go to Administrative Tools -> Local Security Policy -> Local Policies -> Security Option, and change "Network security: LAN Manager authentication level" to "LM & NTLM - Use NTLMv2 session if negotiated".

More info that I can't verify (not a Squid user) can be found in :
Getting Squid to authenticate with kerberos and Windows 2008/2003/7/XP

Solution 2:

In case anyone still needs a solution: I use a software called Proxifier. It forces all applications to use any proxy server you may be connected to. All you need to do is set up the proxy network details, including authentication credentials and you're good to go. So you might find a lot of applications that require an internet connection but have no option of configuring the details of this internet connection. If you're on a proxy, Proxifier will solve these problems most of the time.