fixed issue in Firefox

lib/single-file/doc-helper.js

@@ -83,18 +83,20 @@ this.docHelper = this.docHelper || (() => {
 		const elements = Array.from(element.childNodes).filter(node => node.nodeType == Node.ELEMENT_NODE);
 		elements.forEach(element => markHiddenElements(win, element, markerRemovedContent, markerRemovedCandidate));
 		if (elements.length) {
-			let hiddenElement = !IGNORED_TAG_NAMES.includes(element.nodeName);
-			if (hiddenElement) {
-				hiddenElement = !elements.find(element => (!(element instanceof win.HTMLElement) || element.getAttribute(markerRemovedCandidate) !== ""));
-				hiddenElement = hiddenElement && element.hidden || (element.style && (element.style.display == "none" || element.style.opacity == "0" || element.style.visibility == "hidden"));
-			}
-			if (!hiddenElement) {
-				const boundingRect = element.getBoundingClientRect();
-				hiddenElement = !boundingRect.width && !boundingRect.height;
-			}
-			if (hiddenElement) {
-				element.setAttribute(markerRemovedCandidate, "");
-				elements.forEach(element => element.setAttribute(markerRemovedContent, ""));
+			if (element instanceof win.HTMLElement) {
+				let hiddenElement = !IGNORED_TAG_NAMES.includes(element.nodeName);
+				if (hiddenElement) {
+					hiddenElement = !elements.find(element => (!(element instanceof win.HTMLElement) || element.getAttribute(markerRemovedCandidate) !== ""));
+					hiddenElement = hiddenElement && element.hidden || (element.style && (element.style.display == "none" || element.style.opacity == "0" || element.style.visibility == "hidden"));
+				}
+				if (!hiddenElement) {
+					const boundingRect = element.getBoundingClientRect();
+					hiddenElement = !boundingRect.width && !boundingRect.height;
+				}
+				if (hiddenElement) {
+					element.setAttribute(markerRemovedCandidate, "");
+					elements.forEach(element => element.setAttribute(markerRemovedContent, ""));
+				}
 			}
 		} else {
 			element.setAttribute(markerRemovedCandidate, "");