I know exactly what the problem is now. HURRAY!!! I actually am posting this from the latest nightly of mozilla from behind a msproxy II proxy. So everyone rejoice. If we look closely at what the difference is between the response that Jake's friendly firewall gives when authentication is required, i.e.: HTTP/1.0 407 Proxy authentication required Proxy-Authenticate: Basic realm="" and the response that MSProxy II gives, i.e.: HTTP/1.1 407 Proxy authentication required Proxy-Authenticate: NTLM Proxy-Authenticate: Basic realm="192.168.11.5" Content-Length: 503 Content-Type: text/html . . . We see that MSProxy is giving TWO authenticate lines. Now, I don't know what NTLM is, but when I modified my proxy to excise that line, everything just started working. So I am attaching the modified proxy, which is quite a hack, I apologize, but it works for me. Mozilla ends up seeing, instead of the above transcript, something like HTTP/1.1 407 Proxy authentication required Proxy-Authenticate: Basic realm="192.168.11.5" Content-Length: 503 Content-Type: text/html . . . To use the hack, run something like ./proxy msproxy.ccs.com 765 80 from the command line, where msproxy.ccs.com is the MS proxy you must go through 765 is what ./proxy is listening to for connections 80 is the port that your MS proxy uses Then set your proxy as localhost:765 I make no gaurantee that this will work on your machine, but it works for me.