Bug 42194: Fix blank net error page on failed DNS resolution with active proxy.

toolkit/actors/NetErrorChild.sys.mjs

@@ -133,12 +133,15 @@ export class NetErrorChild extends RemotePageChild {
         shortDesc.appendChild(span);
       },
     };
-
+    try {
       Services.uriFixup.checkHost(
         info.fixedURI,
         onLookupCompleteListener,
         this.document.nodePrincipal.originAttributes
       );
+    } catch (e) {
+      // DNS resolution may fail synchronously if forbidden by proxy
+    }
   }
 
   // Get the header from the http response of the failed channel. This function