diff --git a/netwerk/base/src/nsSimpleURI.cpp b/netwerk/base/src/nsSimpleURI.cpp
index 5cf48fc336fa..4d2df0f2d130 100644
--- a/netwerk/base/src/nsSimpleURI.cpp
+++ b/netwerk/base/src/nsSimpleURI.cpp
@@ -54,6 +54,7 @@
 #include "nsEscape.h"
 #include "nsNetError.h"
 #include "nsIProgrammingLanguage.h"
+#include "mozilla/Util.h" // for DebugOnly
 
 static NS_DEFINE_CID(kThisSimpleURIImplementationCID,
                      NS_THIS_SIMPLEURI_IMPLEMENTATION_CID);
@@ -226,7 +227,7 @@ nsSimpleURI::SetSpec(const nsACString &aSpec)
         return NS_ERROR_MALFORMED_URI;
 
     mScheme.Truncate();
-    PRInt32 n = spec.Left(mScheme, colonPos);
+    mozilla::DebugOnly<PRInt32> n = spec.Left(mScheme, colonPos);
     NS_ASSERTION(n == colonPos, "Left failed");
     ToLowerCase(mScheme);
 
diff --git a/netwerk/cache/nsCacheService.cpp b/netwerk/cache/nsCacheService.cpp
index 49902cc5419a..0bbba7268077 100644
--- a/netwerk/cache/nsCacheService.cpp
+++ b/netwerk/cache/nsCacheService.cpp
@@ -72,6 +72,7 @@
 #include "nsIPrivateBrowsingService.h"
 #include "nsNetCID.h"
 #include <math.h>  // for log()
+#include "mozilla/Util.h" // for DebugOnly
 #include "mozilla/Services.h"
 
 #include "mozilla/FunctionTimer.h"
@@ -1818,7 +1819,7 @@ nsCacheService::EnsureEntryHasDevice(nsCacheEntry * entry)
             if (predictedDataSize != -1 &&
                 entry->StoragePolicy() != nsICache::STORE_ON_DISK_AS_FILE &&
                 mDiskDevice->EntryIsTooBig(predictedDataSize)) {
-                nsresult rv = nsCacheService::DoomEntry(entry);
+                DebugOnly<nsresult> rv = nsCacheService::DoomEntry(entry);
                 NS_ASSERTION(NS_SUCCEEDED(rv),"DoomEntry() failed.");
                 return nsnull;
             }
@@ -1841,7 +1842,7 @@ nsCacheService::EnsureEntryHasDevice(nsCacheEntry * entry)
             // Bypass the cache if Content-Length says entry will be too big
             if (predictedDataSize != -1 &&
                 mMemoryDevice->EntryIsTooBig(predictedDataSize)) {
-                nsresult rv = nsCacheService::DoomEntry(entry);
+                DebugOnly<nsresult> rv = nsCacheService::DoomEntry(entry);
                 NS_ASSERTION(NS_SUCCEEDED(rv),"DoomEntry() failed.");
                 return nsnull;
             }
diff --git a/netwerk/cookie/nsCookieService.cpp b/netwerk/cookie/nsCookieService.cpp
index 0fbc01dd3006..ac28a538f0b1 100644
--- a/netwerk/cookie/nsCookieService.cpp
+++ b/netwerk/cookie/nsCookieService.cpp
@@ -83,6 +83,7 @@
 #include "nsNetCID.h"
 #include "mozilla/storage.h"
 #include "mozilla/FunctionTimer.h"
+#include "mozilla/Util.h" // for DebugOnly
 
 using namespace mozilla::net;
 
@@ -1945,7 +1946,7 @@ nsCookieService::CancelAsyncRead(PRBool aPurgeReadSet)
   // Cancel the pending read, kill the read listener, and empty the array
   // of data already read in on the background thread.
   mDefaultDBState->readListener->Cancel();
-  nsresult rv = mDefaultDBState->pendingRead->Cancel();
+  mozilla::DebugOnly<nsresult> rv = mDefaultDBState->pendingRead->Cancel();
   NS_ASSERT_SUCCESS(rv);
 
   mDefaultDBState->stmtReadDomain = nsnull;