--- a/netwerk/protocol/http/nsHttpConnectionMgr.cpp
+++ b/netwerk/protocol/http/nsHttpConnectionMgr.cpp
@@ -2273,17 +2273,22 @@ nsHttpConnectionMgr::OnMsgReschedTransac
 void nsHttpConnectionMgr::OnMsgUpdateClassOfServiceOnTransaction(int32_t arg, ARefBase *param)
 {
     MOZ_ASSERT(OnSocketThread(), "not on socket thread");
     LOG(("nsHttpConnectionMgr::OnMsgUpdateClassOfServiceOnTransaction [trans=%p]\n", param));
 
     uint32_t cos = static_cast<uint32_t>(arg);
     nsHttpTransaction *trans = static_cast<nsHttpTransaction *>(param);
 
+    uint32_t previous = trans->ClassOfService();
     trans->SetClassOfService(cos);
+
+    if ((previous ^ cos) & (NS_HTTP_LOAD_AS_BLOCKING | NS_HTTP_LOAD_UNBLOCKED)) {
+        Unused << RescheduleTransaction(trans, trans->Priority());
+    }
 }
 
 void
 nsHttpConnectionMgr::OnMsgCancelTransaction(int32_t reason, ARefBase *param)
 {
     MOZ_ASSERT(OnSocketThread(), "not on socket thread");
     LOG(("nsHttpConnectionMgr::OnMsgCancelTransaction [trans=%p]\n", param));