added missing selector

lib/frame-tree/content/frame-tree.js

@@ -91,7 +91,7 @@ this.FrameTree = (() => {
 	function initRequest(message) {
 		FrameTree.windowId = message.windowId;
 		FrameTree.index = message.index;
-		const frameElements = document.querySelectorAll("frame, object[type=\"text/html\"][data]");
+		const frameElements = document.querySelectorAll("iframe, frame, object[type=\"text/html\"][data]");
 		if (frameElements.length) {
 			setFramesWinId(MESSAGE_PREFIX, frameElements, FrameTree.index, FrameTree.windowId, window);
 		} else {