--- a/uriloader/base/nsURILoader.cpp
+++ b/uriloader/base/nsURILoader.cpp
@@ -534,18 +534,18 @@ nsresult nsDocumentOpenInfo::DispatchCon
   }
 
   // Before dispatching to the external helper app service, check for an HTTP
   // error page.  If we got one, we don't want to handle it with a helper app,
   // really.
   nsCOMPtr<nsIHttpChannel> httpChannel(do_QueryInterface(request));
   if (httpChannel) {
     bool requestSucceeded;
-    httpChannel->GetRequestSucceeded(&requestSucceeded);
-    if (!requestSucceeded) {
+    rv = httpChannel->GetRequestSucceeded(&requestSucceeded);
+    if (NS_FAILED(rv) || !requestSucceeded) {
       // returning error from OnStartRequest will cancel the channel
       return NS_ERROR_FILE_NOT_FOUND;
     }
   }
   
   // Sixth step:
   //
   // All attempts to dispatch this content have failed.  Just pass it off to

--- a/uriloader/exthandler/nsExternalHelperAppService.cpp
+++ b/uriloader/exthandler/nsExternalHelperAppService.cpp
@@ -787,17 +787,17 @@ NS_IMETHODIMP nsExternalHelperAppService
     channel->GetContentDispositionFilename(fileName);
 
     // Check if we have a POST request, in which case we don't want to use
     // the url's extension
     bool allowURLExt = true;
     nsCOMPtr<nsIHttpChannel> httpChan = do_QueryInterface(channel);
     if (httpChan) {
       nsAutoCString requestMethod;
-      httpChan->GetRequestMethod(requestMethod);
+      Unused << httpChan->GetRequestMethod(requestMethod);
       allowURLExt = !requestMethod.EqualsLiteral("POST");
     }
 
     // Check if we had a query string - we don't want to check the URL
     // extension if a query is present in the URI
     // If we already know we don't want to check the URL extension, don't
     // bother checking the query
     if (uri && allowURLExt) {
@@ -1669,18 +1669,18 @@ NS_IMETHODIMP nsExternalAppHandler::OnSt
   // retarget all load notifications to our docloader instead of the original window's docloader...
   RetargetLoadNotifications(request);
 
   // Check to see if there is a refresh header on the original channel.
   if (mOriginalChannel) {
     nsCOMPtr<nsIHttpChannel> httpChannel(do_QueryInterface(mOriginalChannel));
     if (httpChannel) {
       nsAutoCString refreshHeader;
-      httpChannel->GetResponseHeader(NS_LITERAL_CSTRING("refresh"),
-                                     refreshHeader);
+      Unused << httpChannel->GetResponseHeader(NS_LITERAL_CSTRING("refresh"),
+                                               refreshHeader);
       if (!refreshHeader.IsEmpty()) {
         mShouldCloseWindow = false;
       }
     }
   }
 
   // Close the underlying DOMWindow if there is no refresh header
   // and it was opened specifically for the download
@@ -1720,17 +1720,18 @@ NS_IMETHODIMP nsExternalAppHandler::OnSt
     SendStatusChange(kWriteError, transferError, request, path);
 
     return NS_OK;
   }
 
   // Inform channel it is open on behalf of a download to prevent caching.
   nsCOMPtr<nsIHttpChannelInternal> httpInternal = do_QueryInterface(aChannel);
   if (httpInternal) {
-    httpInternal->SetChannelIsForDownload(true);
+    rv = httpInternal->SetChannelIsForDownload(true);
+    MOZ_ASSERT(NS_SUCCEEDED(rv));
   }
 
   // now that the temp file is set up, find out if we need to invoke a dialog
   // asking the user what they want us to do with this content...
 
   // We can get here for three reasons: "can't handle", "sniffed type", or
   // "server sent content-disposition:attachment".  In the first case we want
   // to honor the user's "always ask" pref; in the other two cases we want to

--- a/uriloader/prefetch/nsOfflineCacheUpdate.cpp
+++ b/uriloader/prefetch/nsOfflineCacheUpdate.cpp
@@ -187,20 +187,22 @@ nsManifestCheck::Begin()
                        nsIRequest::LOAD_BYPASS_CACHE);
 
     NS_ENSURE_SUCCESS(rv, rv);
 
     // configure HTTP specific stuff
     nsCOMPtr<nsIHttpChannel> httpChannel =
         do_QueryInterface(mChannel);
     if (httpChannel) {
-        httpChannel->SetReferrer(mReferrerURI);
-        httpChannel->SetRequestHeader(NS_LITERAL_CSTRING("X-Moz"),
-                                      NS_LITERAL_CSTRING("offline-resource"),
-                                      false);
+        rv = httpChannel->SetReferrer(mReferrerURI);
+        MOZ_ASSERT(NS_SUCCEEDED(rv));
+        rv = httpChannel->SetRequestHeader(NS_LITERAL_CSTRING("X-Moz"),
+                                           NS_LITERAL_CSTRING("offline-resource"),
+                                           false);
+        MOZ_ASSERT(NS_SUCCEEDED(rv));
     }
 
     return mChannel->AsyncOpen2(this);
 }
 
 //-----------------------------------------------------------------------------
 // nsManifestCheck <public>
 //-----------------------------------------------------------------------------
@@ -396,20 +398,22 @@ nsOfflineCacheUpdateItem::OpenChannel(ns
     // Set the new application cache as the target for write.
     rv = appCacheChannel->SetApplicationCacheForWrite(mApplicationCache);
     NS_ENSURE_SUCCESS(rv, rv);
 
     // configure HTTP specific stuff
     nsCOMPtr<nsIHttpChannel> httpChannel =
         do_QueryInterface(mChannel);
     if (httpChannel) {
-        httpChannel->SetReferrer(mReferrerURI);
-        httpChannel->SetRequestHeader(NS_LITERAL_CSTRING("X-Moz"),
-                                      NS_LITERAL_CSTRING("offline-resource"),
-                                      false);
+        rv = httpChannel->SetReferrer(mReferrerURI);
+        MOZ_ASSERT(NS_SUCCEEDED(rv));
+        rv = httpChannel->SetRequestHeader(NS_LITERAL_CSTRING("X-Moz"),
+                                           NS_LITERAL_CSTRING("offline-resource"),
+                                           false);
+        MOZ_ASSERT(NS_SUCCEEDED(rv));
     }
 
     rv = mChannel->AsyncOpen2(this);
     NS_ENSURE_SUCCESS(rv, rv);
 
     mUpdate = aUpdate;
 
     mState = LoadStatus::REQUESTED;
@@ -574,19 +578,20 @@ nsOfflineCacheUpdateItem::AsyncOnChannel
         LOG(("rejected: redirected to a different scheme\n"));
         return NS_ERROR_ABORT;
     }
 
     // HTTP request headers are not automatically forwarded to the new channel.
     nsCOMPtr<nsIHttpChannel> httpChannel = do_QueryInterface(aNewChannel);
     NS_ENSURE_STATE(httpChannel);
 
-    httpChannel->SetRequestHeader(NS_LITERAL_CSTRING("X-Moz"),
-                                  NS_LITERAL_CSTRING("offline-resource"),
-                                  false);
+    rv = httpChannel->SetRequestHeader(NS_LITERAL_CSTRING("X-Moz"),
+                                       NS_LITERAL_CSTRING("offline-resource"),
+                                       false);
+    MOZ_ASSERT(NS_SUCCEEDED(rv));
 
     mChannel = aNewChannel;
 
     cb->OnRedirectVerifyCallback(NS_OK);
     return NS_OK;
 }
 
 nsresult

--- a/uriloader/prefetch/nsPrefetchService.cpp
+++ b/uriloader/prefetch/nsPrefetchService.cpp
@@ -145,21 +145,22 @@ nsPrefetchNode::OpenChannel()
                                         nsICachingChannel::LOAD_ONLY_IF_MODIFIED);
 
     NS_ENSURE_SUCCESS(rv, rv);
 
     // configure HTTP specific stuff
     nsCOMPtr<nsIHttpChannel> httpChannel =
         do_QueryInterface(mChannel);
     if (httpChannel) {
-        httpChannel->SetReferrerWithPolicy(mReferrerURI, referrerPolicy);
-        httpChannel->SetRequestHeader(
-            NS_LITERAL_CSTRING("X-Moz"),
-            NS_LITERAL_CSTRING("prefetch"),
-            false);
+        rv = httpChannel->SetReferrerWithPolicy(mReferrerURI, referrerPolicy);
+        MOZ_ASSERT(NS_SUCCEEDED(rv));
+        rv = httpChannel->SetRequestHeader(NS_LITERAL_CSTRING("X-Moz"),
+                                           NS_LITERAL_CSTRING("prefetch"),
+                                           false);
+        MOZ_ASSERT(NS_SUCCEEDED(rv));
     }
 
     // Reduce the priority of prefetch network requests.
     nsCOMPtr<nsISupportsPriority> priorityChannel = do_QueryInterface(mChannel);
     if (priorityChannel) {
       priorityChannel->AdjustPriority(nsISupportsPriority::PRIORITY_LOWEST);
     }
 
@@ -334,19 +335,20 @@ nsPrefetchNode::AsyncOnChannelRedirect(n
             return NS_ERROR_ABORT;
         }
     }
 
     // HTTP request headers are not automatically forwarded to the new channel.
     nsCOMPtr<nsIHttpChannel> httpChannel = do_QueryInterface(aNewChannel);
     NS_ENSURE_STATE(httpChannel);
 
-    httpChannel->SetRequestHeader(NS_LITERAL_CSTRING("X-Moz"),
-                                  NS_LITERAL_CSTRING("prefetch"),
-                                  false);
+    rv = httpChannel->SetRequestHeader(NS_LITERAL_CSTRING("X-Moz"),
+                                       NS_LITERAL_CSTRING("prefetch"),
+                                       false);
+    MOZ_ASSERT(NS_SUCCEEDED(rv));
 
     // Assign to mChannel after we get notification about success of the
     // redirect in OnRedirectResult.
     mRedirectChannel = aNewChannel;
 
     callback->OnRedirectVerifyCallback(NS_OK);
     return NS_OK;
 }