The FoxyProxy relative:// scheme (schemes are sometimes confused with protocols) provides a means to specify relative URLs on local and network resources. It is a feature specific to FoxyProxy. This is most useful when a PAC file is located on a removable drive because the removable drive can be assigned different drive letters or mount points, depending on the computer in use or logged-in user.
relative:// URLs are comprised of one or more paths followed by a filename, just like http:// or file:// URLs. However, with relative:// URLs, one or more of the paths can consist of special substitution strings. These strings are substituted with file system paths when FoxyProxy must resolve the relative:// URL to an absolute resource.
The complete list of substitution strings is here. THEY ARE CASE-SENSITIVE. To construct relateive:// URLs, simply use the substitution string in the appropriate place. Some examples:
Filepath MyPACs/proxy1.pac in the Firefox profile directory. The Firefox profile directory is a relative directory when using Portable Firefox because its location may change when Portable Firefox is used on different computers (e.g., drive letter changes).
Filepath subdir/foo.pac on the Windows desktop. The Windows desktop is a relative directory because its location varies depending upon the logged on user.