--- a/toolkit/mozapps/extensions/AddonManager.jsm
+++ b/toolkit/mozapps/extensions/AddonManager.jsm
@@ -1996,18 +1996,19 @@ var AddonManagerInternal = {
   },
 
   startInstall(browser, url, install) {
     this.installNotifyObservers("addon-install-started", browser, url, install);
 
     // Local installs may already be in a failed state in which case
     // we won't get any further events, detect those cases now.
     if (install.state == AddonManager.STATE_DOWNLOADED && install.addon.appDisabled) {
-          this.installNotifyObservers("addon-install-failed", browser, url, install);
-          return;
+      install.cancel();
+      this.installNotifyObservers("addon-install-failed", browser, url, install);
+      return;
     }
 
     let self = this;
     let listener = {
       onDownloadCancelled() {
         install.removeListener(listener);
       },