I’ve had this weird proxy authentication issue that prevented me from connecting to the internet via Firefox on a Mac from behind a corporate firewall.
I switched a couple of authentication options for Firefox as shown below to solve the problem.
Open an instance of Firefox and type in “about:config” in the address bar. Click the “I’ll be careful, I promise” button.
In the Search field enter “network.negotiate” and select the “network.negotiate-auth.allow-proxies” settings row. Double-click the row to change the Value to “false”.
If you’re using a Microsoft ISA proxy server, this setting effectively disables SPNEGO client-side and gets you through.
In the Search field enter “network.automatic” and select the “network.automatic-ntlm.allow-proxies” settings row. Double-click the row to change the Value to “false”.
If you haven’t attached your machine to a Windows domain, this setting prevents Firefox attempting to silently “single sign-on” to the ISA proxy using your non-domain credentials (and therefore failing the authentication challenge).
Go to Firefox -> Preferences -> Network -> Settings and click to select the “Auto-detect proxy settings for this network” radio button option.
In the event that you need to use a Cisco VPN client to connect remotely, the latest version of the Cisco VPN client (v3.1 as of this writing) must be used. The above settings (Step 1 – 4) should work seamlessly, connecting you to internal and external web sites even when you’re on the VPN.