--- a/netwerk/base/src/nsIOService.cpp
+++ b/netwerk/base/src/nsIOService.cpp
@@ -545,17 +545,27 @@ nsIOService::NewChannelFromURI(nsIURI *a
     if (protoFlags & nsIProtocolHandler::ALLOWS_PROXY) {
         nsCOMPtr<nsIProxyInfo> pi;
         if (!mProxyService) {
             mProxyService = do_GetService(NS_PROTOCOLPROXYSERVICE_CONTRACTID);
             if (!mProxyService)
                 NS_WARNING("failed to get protocol proxy service");
         }
         if (mProxyService) {
-            rv = mProxyService->Resolve(aURI, 0, getter_AddRefs(pi));
+            PRUint32 flags = 0;
+            if (scheme.EqualsLiteral("http") || scheme.EqualsLiteral("https"))
+                flags = nsIProtocolProxyService::RESOLVE_NON_BLOCKING;
+            rv = mProxyService->Resolve(aURI, flags, getter_AddRefs(pi));
+            if (rv == NS_BASE_STREAM_WOULD_BLOCK) {
+                // Use an UNKNOWN proxy to defer resolution and avoid blocking.
+                rv = mProxyService->NewProxyInfo(NS_LITERAL_CSTRING("unknown"),
+                                                 NS_LITERAL_CSTRING(""),
+                                                 -1, 0, 0, nsnull,
+                                                 getter_AddRefs(pi));
+            }
             if (NS_FAILED(rv))
                 pi = nsnull;
         }
         if (pi) {
             nsCAutoString type;
             if (NS_SUCCEEDED(pi->GetType(type)) && type.EqualsLiteral("http")) {
                 // we are going to proxy this channel using an http proxy
                 rv = GetProtocolHandler("http", getter_AddRefs(handler));

--- a/netwerk/base/src/nsProtocolProxyService.cpp
+++ b/netwerk/base/src/nsProtocolProxyService.cpp
@@ -918,17 +918,18 @@ nsProtocolProxyService::NewProxyInfo(con
                                      PRUint32 aFailoverTimeout,
                                      nsIProxyInfo *aFailoverProxy,
                                      nsIProxyInfo **aResult)
 {
     static const char *types[] = {
         kProxyType_HTTP,
         kProxyType_SOCKS,
         kProxyType_SOCKS4,
-        kProxyType_DIRECT
+        kProxyType_DIRECT,
+        kProxyType_UNKNOWN
     };
 
     // resolve type; this allows us to avoid copying the type string into each
     // proxy info instance.  we just reference the string literals directly :)
     const char *type = nsnull;
     for (PRUint32 i=0; i<NS_ARRAY_LENGTH(types); ++i) {
         if (aType.LowerCaseEqualsASCII(types[i])) {
             type = types[i];