Bug 597911. Give 64-bit builds on Windows a different application id. r=rstrong

browser/installer/windows/nsis/defines.nsi.in

@@ -8,7 +8,12 @@
 !define AppVendor             "Mozilla"
 !define AppName               "Firefox"
 !define AppVersion            "@APP_VERSION@"
+#ifdef HAVE_64BIT_OS
+// differentiate 64-bit builds, we do the same in widget.
+!define AppUserModelID        "${AppVendor}.${AppName}.${AppVersion}.Win64"
+#else
 !define AppUserModelID        "${AppVendor}.${AppName}.${AppVersion}"
+#endif
 !define GREVersion            @MOZILLA_VERSION@
 !define AB_CD                 "@AB_CD@"
 

widget/src/windows/WinTaskbar.cpp

@@ -293,7 +293,15 @@ WinTaskbar::GetAppUserModelID(nsAString & aDefaultGroupId) {
     AppendASCIItoUTF16(val, aDefaultGroupId);
   }
 
-  return aDefaultGroupId.IsEmpty() ? PR_FALSE : PR_TRUE;
+  if (aDefaultGroupId.IsEmpty())
+    return PR_FALSE;
+
+  // Differentiate 64-bit builds
+#if defined(_WIN64)
+  aDefaultGroupId.AppendLiteral(".Win64");
+#endif
+
+  return PR_TRUE;
 }
 
 /* readonly attribute AString defaultGroupId; */