Was a great ext before Quantum.
But now it can't use "PAC" or "System settings", why?
Look at this ext - https://addons.mozilla.org/en-US/firefox/addon/proxy-switcher-and-manager/ - it can do it! Why don't you look at its src or contact to author, and fix your issue?
I'm now using proxyswitchyomega (have the same issue as foxyproxy) + "proxy switcher" (works well, but does not support patterns). When no pattern match, omega use "direct", and then pac/system from "proxy switcher' steps in. It works good, but it would be great to do it with one addon.