Currently, the only mechanism for addon to change proxy (apart from modifying the prefs) is adding proxy filter, and the interface of proxy filter is designed that it can only return proxy info synchronously. However, proxy decision may rely on the IP address of the target instead of only the url. In that case, the filter have to synchronously resolve the address because of the limitation above, which can block the requesting thread. It is undesired. OTOH, for PAC file, we do that in a separate thread, and make the dnsResolve() function be handled asynchronously. It is funny that something we can do in pac file cannot be done via internal interface for addon. So I suggest we add API to allow having async proxy filter.

Bulk change to priority: https://bugzilla.mozilla.org/show_bug.cgi?id=1399258