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diff --git a/toolkit/components/reader/Readability-readerable.js b/toolkit/components/reader/Readability-readerable.js
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+++ b/toolkit/components/reader/Readability-readerable.js
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+/* eslint-env es6:false */
+/* globals exports */
+/*
+ * DO NOT MODIFY THIS FILE DIRECTLY!
+ *
+ * This is a shared library that is maintained in an external repo:
+ * https://github.com/mozilla/readability
+ */
+
+/*
+ * Copyright (c) 2010 Arc90 Inc
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+/*
+ * This code is heavily based on Arc90's readability.js (1.7.1) script
+ * available at: http://code.google.com/p/arc90labs-readability
+ */
+
+var REGEXPS = {
+  // NOTE: These two regular expressions are duplicated in
+  // Readability.js. Please keep both copies in sync.
+  unlikelyCandidates: /-ad-|banner|breadcrumbs|combx|comment|community|cover-wrap|disqus|extra|foot|header|legends|menu|related|remark|replies|rss|shoutbox|sidebar|skyscraper|social|sponsor|supplemental|ad-break|agegate|pagination|pager|popup|yom-remote/i,
+  okMaybeItsACandidate: /and|article|body|column|main|shadow/i,
+};
+
+function isNodeVisible(node) {
+  // Have to null-check node.style to deal with SVG and MathML nodes.
+  return (!node.style || node.style.display != "none") && !node.hasAttribute("hidden");
+}
+
+/**
+ * Decides whether or not the document is reader-able without parsing the whole thing.
+ *
+ * @return boolean Whether or not we suspect Readability.parse() will suceeed at returning an article object.
+ */
+function isProbablyReaderable(doc, isVisible) {
+  if (!isVisible) {
+    isVisible = isNodeVisible;
+  }
+
+  var nodes = doc.querySelectorAll("p, pre");
+
+  // Get <div> nodes which have <br> node(s) and append them into the `nodes` variable.
+  // Some articles' DOM structures might look like
+  // <div>
+  //   Sentences<br>
+  //   <br>
+  //   Sentences<br>
+  // </div>
+  var brNodes = doc.querySelectorAll("div > br");
+  if (brNodes.length) {
+    var set = new Set(nodes);
+    [].forEach.call(brNodes, function(node) {
+      set.add(node.parentNode);
+    });
+    nodes = Array.from(set);
+  }
+
+  var score = 0;
+  // This is a little cheeky, we use the accumulator 'score' to decide what to return from
+  // this callback:
+  return [].some.call(nodes, function(node) {
+    if (!isVisible(node))
+      return false;
+
+    var matchString = node.className + " " + node.id;
+    if (REGEXPS.unlikelyCandidates.test(matchString) &&
+        !REGEXPS.okMaybeItsACandidate.test(matchString)) {
+      return false;
+    }
+
+    if (node.matches("li p")) {
+      return false;
+    }
+
+    var textContentLength = node.textContent.trim().length;
+    if (textContentLength < 140) {
+      return false;
+    }
+
+    score += Math.sqrt(textContentLength - 140);
+
+    if (score > 20) {
+      return true;
+    }
+    return false;
+  });
+}
+
+if (typeof exports === "object") {
+  exports.isProbablyReaderable = isProbablyReaderable;
+}