The path and query components should only be stripped for https:// URLs, and only when the preference network.proxy.autoconfig_url.include_path is set to false. Reference: https://developer.mozilla.org/en-US/docs/Web/HTTP/Proxy_servers_and_tunneling/Proxy_Auto-Configuration_(PAC)_file#Parameters
Since our FindProxyForPAC() implementation is non-standard (see bug 1381290 and others), we should not fix this. It is more useful for addons to have the entire URL at all times, rather than some of the URL sometimes and all of the URL at other times. Having the URL at all times is also analogous to what legacy addons had with nsIProtocolProxyFilter.applyFilter(). Since addons are trusted, there is no reason to hide path and query components or any other part of the URL from the addon. The stripping of this info for traditional, server-hosted PAC files may have had to do with security precautions. Since our FindProxyForPAC() is hosted within the locally-hosted addon itself, there's no added security risk by providing this info. Indeed, it is also available to the addon via webRequest API listeners.
Currently the path component of the URL isn't provided at all, and I think you're suggesting that it should be provided for all URLs, correct? If so, we would still have to make that change.