I was reviewing again the webidl:

+[NavigatorProperty="mozDownloadManager",
+ JSImplementation="@mozilla.org/downloads/manager;1",
+ Pref="dom.mozDownloads.enabled"]
+interface DOMDownloadManager : EventTarget {
+  // This promise returns an array of downloads with all the current
+  // download objects.
+  Promise getDownloads();
+
+  // Removes one download from the downloads set. Returns a promise resolved
+  // with the finalized download.
+  Promise remove(DOMDownload download);
+
+  // Removes all the completed downloads from the set.
+  Promise clearAllDone();
+
+  // Fires when a new download starts.
+  attribute EventHandler ondownloadstart;
+};

and we pass a download to remove