How can I tell Firefox where to find my proxy auto configuration file without using as drive letter to do so? I have a few hundred client computers on two independent networks. Most of them dont have Firefox installed so I carry Portable Firefox on a thumb drive for use when fixing the clients PC involves a foray into the WWW. The problem is that the two networks use different proxies. Plus, I use the same thumb drive at CyberCafes and at other times when there is no proxy involved. I have written a proxy auto config script (a .pac file) that tests to determine which network is present (if any) and sets the proxy accordingly. But the drive letter for the thumb drive changes from PC to PC so I need to designate the proxy.pac files location without using a drive letter. As a stopgap, Im using a batch file to fire up the Portable Firefox that puts a copy of the proxy.pac at the system root before launching the browser. Thats not an especially elegant solution, plus I'm sure I'll eventually encounter a public PC that will deny me access to its root directory. Not to mention Id prefer to do it entirely in the browser configuration, if possible. Any ideas?