diff --git a/toolkit/mozapps/update/UpdateService.jsm b/toolkit/mozapps/update/UpdateService.jsm
index 4788151add89..0ea39ea299cc 100644
--- a/toolkit/mozapps/update/UpdateService.jsm
+++ b/toolkit/mozapps/update/UpdateService.jsm
@@ -1942,6 +1942,38 @@ function updateIsAtLeastAsOldAs(update, version, buildID) {
   );
 }
 
+/**
+ * This returns true if the passed update is the same version or older than
+ * currently installed Firefox version.
+ */
+function updateIsAtLeastAsOldAsCurrentVersion(update) {
+  return updateIsAtLeastAsOldAs(
+    update,
+    Services.appinfo.version,
+    Services.appinfo.appBuildID
+  );
+}
+
+/**
+ * This returns true if the passed update is the same version or older than
+ * the update that we have already downloaded (UpdateManager.readyUpdate).
+ * Returns false if no update has already been downloaded.
+ */
+function updateIsAtLeastAsOldAsReadyUpdate(update) {
+  if (
+    !UM.readyUpdate ||
+    !UM.readyUpdate.appVersion ||
+    !UM.readyUpdate.buildID
+  ) {
+    return false;
+  }
+  return updateIsAtLeastAsOldAs(
+    update,
+    UM.readyUpdate.appVersion,
+    UM.readyUpdate.buildID
+  );
+}
+
 /**
  * Update Patch
  * @param   patch
@@ -3429,20 +3461,35 @@ UpdateService.prototype = {
     updates.forEach(function(aUpdate) {
       // Ignore updates for older versions of the application and updates for
       // the same version of the application with the same build ID.
-      if (
-        vc.compare(aUpdate.appVersion, Services.appinfo.version) < 0 ||
-        (vc.compare(aUpdate.appVersion, Services.appinfo.version) == 0 &&
-          aUpdate.buildID == Services.appinfo.appBuildID)
-      ) {
+      if (updateIsAtLeastAsOldAsCurrentVersion(aUpdate)) {
         LOG(
           "UpdateService:selectUpdate - skipping update because the " +
-            "update's application version is less than the current " +
+            "update's application version is not greater than the current " +
             "application version"
         );
         lastCheckCode = AUSTLMY.CHK_UPDATE_PREVIOUS_VERSION;
         return;
       }
 
+      if (updateIsAtLeastAsOldAsReadyUpdate(aUpdate)) {
+        LOG(
+          "UpdateService:selectUpdate - skipping update because the " +
+            "update's application version is not greater than that of the " +
+            "currently downloaded update"
+        );
+        lastCheckCode = AUSTLMY.CHK_UPDATE_PREVIOUS_VERSION;
+        return;
+      }
+
+      if (UM.readyUpdate && !getPatchOfType(aUpdate, "partial")) {
+        LOG(
+          "UpdateService:selectUpdate - skipping update because no partial " +
+            "patch is available and an update has already been downloaded."
+        );
+        lastCheckCode = AUSTLMY.CHK_NO_PARTIAL_PATCH;
+        return;
+      }
+
       switch (aUpdate.type) {
         case "major":
           if (!majorUpdate) {
@@ -3893,13 +3940,7 @@ UpdateService.prototype = {
     // build ID. If we already have an update ready, we want to apply those
     // same checks against the version of the ready update, so that we don't
     // download an update that isn't newer than the one we already have.
-    if (
-      updateIsAtLeastAsOldAs(
-        update,
-        Services.appinfo.version,
-        Services.appinfo.appBuildID
-      )
-    ) {
+    if (updateIsAtLeastAsOldAsCurrentVersion(update)) {
       LOG(
         "UpdateService:downloadUpdate - canceling download of update since " +
           "it is for an earlier or same application version and build ID.\n" +
@@ -3918,16 +3959,7 @@ UpdateService.prototype = {
       cleanupDownloadingUpdate();
       return false;
     }
-    if (
-      UM.readyUpdate &&
-      UM.readyUpdate.appVersion &&
-      UM.readyUpdate.buildID &&
-      updateIsAtLeastAsOldAs(
-        update,
-        UM.readyUpdate.appVersion,
-        UM.readyUpdate.buildID
-      )
-    ) {
+    if (updateIsAtLeastAsOldAsReadyUpdate(update)) {
       LOG(
         "UpdateService:downloadUpdate - not downloading update because the " +
           "update that's already been downloaded is the same version or " +
diff --git a/toolkit/mozapps/update/UpdateTelemetry.jsm b/toolkit/mozapps/update/UpdateTelemetry.jsm
index dae76e09acd0..51c25ca051ff 100644
--- a/toolkit/mozapps/update/UpdateTelemetry.jsm
+++ b/toolkit/mozapps/update/UpdateTelemetry.jsm
@@ -101,6 +101,9 @@ var AUSTLMY = {
   // Update check was delayed because another instance of the application is
   // currently running
   CHK_OTHER_INSTANCE: 39,
+  // Cannot yet download update because no partial patch is available and an
+  // update has already been downloaded.
+  CHK_NO_PARTIAL_PATCH: 40,
 
   /**
    * Submit a telemetry ping for the update check result code or a telemetry