diff --git a/content/xul/document/src/nsXULPrototypeCache.cpp b/content/xul/document/src/nsXULPrototypeCache.cpp
index e6b4d13728d3..ef926610150b 100644
--- a/content/xul/document/src/nsXULPrototypeCache.cpp
+++ b/content/xul/document/src/nsXULPrototypeCache.cpp
@@ -778,29 +778,6 @@ nsXULPrototypeCache::StartFastLoad(nsIURI* aURI)
         rv = fastLoadService->NewInputStream(file, getter_AddRefs(objectInput));
 
         if (NS_SUCCEEDED(rv)) {
-            if (gChecksumXULFastLoadFile) {
-                nsCOMPtr<nsIFastLoadReadControl>
-                    readControl(do_QueryInterface(objectInput));
-                if (readControl) {
-                    // Verify checksum, using the fastLoadService's checksum
-                    // cache to avoid computing more than once per session.
-                    PRUint32 checksum;
-                    rv = readControl->GetChecksum(&checksum);
-                    if (NS_SUCCEEDED(rv)) {
-                        PRUint32 verified;
-                        rv = fastLoadService->ComputeChecksum(file,
-                                                               readControl,
-                                                               &verified);
-                        if (NS_SUCCEEDED(rv) && verified != checksum) {
-#ifdef DEBUG
-                            printf("bad FastLoad file checksum\n");
-#endif
-                            rv = NS_ERROR_FAILURE;
-                        }
-                    }
-                }
-            }
-
             if (NS_SUCCEEDED(rv)) {
                 // Get the XUL fastload file version number, which should be
                 // decremented whenever the XUL-specific file format changes
diff --git a/js/src/xpconnect/loader/mozJSComponentLoader.cpp b/js/src/xpconnect/loader/mozJSComponentLoader.cpp
index 3b3aaff353e1..48f9615a7021 100644
--- a/js/src/xpconnect/loader/mozJSComponentLoader.cpp
+++ b/js/src/xpconnect/loader/mozJSComponentLoader.cpp
@@ -959,23 +959,6 @@ mozJSComponentLoader::StartFastLoad(nsIFastLoadService *flSvc)
 
                 nsCOMPtr<nsIFastLoadReadControl>
                     readControl(do_QueryInterface(mFastLoadInput));
-                if (readControl) {
-                    // Verify checksum, using the FastLoadService's
-                    // checksum cache to avoid computing more than once
-                    // per session.
-                    PRUint32 checksum;
-                    rv = readControl->GetChecksum(&checksum);
-                    if (NS_SUCCEEDED(rv)) {
-                        PRUint32 verified;
-                        rv = flSvc->ComputeChecksum(mFastLoadFile,
-                                                    readControl, &verified);
-                        if (NS_SUCCEEDED(rv) && verified != checksum) {
-                            LOG(("Incorrect checksum detected"));
-                            rv = NS_ERROR_FAILURE;
-                        }
-                    }
-                }
-
                 if (NS_SUCCEEDED(rv)) {
                     /* Get the JS bytecode version number and validate it. */
                     PRUint32 version;
diff --git a/xpcom/io/nsFastLoadFile.cpp b/xpcom/io/nsFastLoadFile.cpp
index afba18b2057f..28af3d8ac1a8 100644
--- a/xpcom/io/nsFastLoadFile.cpp
+++ b/xpcom/io/nsFastLoadFile.cpp
@@ -887,6 +887,14 @@ nsFastLoadFileReader::Open()
     if (NS_FAILED(rv))
         return rv;
 
+    PRUint32 checksum;
+    rv = ComputeChecksum(&checksum);
+    if (NS_FAILED(rv))
+        return rv;
+    
+    if (checksum != mHeader.mChecksum)
+        return NS_ERROR_FAILURE;
+
     if (mHeader.mVersion != MFL_FILE_VERSION ||
         mHeader.mFooterOffset == 0 || 
         memcmp(mHeader.mMagic, magic, MFL_FILE_MAGIC_SIZE))