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diff --git a/components/sessionstore/DocumentUtils.jsm b/components/sessionstore/DocumentUtils.jsm
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+++ b/components/sessionstore/DocumentUtils.jsm
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+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this file,
+ * You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+this.EXPORTED_SYMBOLS = [ "DocumentUtils" ];
+
+const Cu = Components.utils;
+const Ci = Components.interfaces;
+
+Cu.import("resource://gre/modules/XPCOMUtils.jsm");
+Cu.import("resource:///modules/sessionstore/XPathGenerator.jsm");
+
+this.DocumentUtils = {
+  /**
+   * Obtain form data for a DOMDocument instance.
+   *
+   * The returned object has 2 keys, "id" and "xpath". Each key holds an object
+   * which further defines form data.
+   *
+   * The "id" object maps element IDs to values. The "xpath" object maps the
+   * XPath of an element to its value.
+   *
+   * @param  aDocument
+   *         DOMDocument instance to obtain form data for.
+   * @return object
+   *         Form data encoded in an object.
+   */
+  getFormData: function DocumentUtils_getFormData(aDocument) {
+    let formNodes = aDocument.evaluate(
+      XPathGenerator.restorableFormNodes,
+      aDocument,
+      XPathGenerator.resolveNS,
+      Ci.nsIDOMXPathResult.UNORDERED_NODE_ITERATOR_TYPE, null
+    );
+
+    let node;
+    let ret = {id: {}, xpath: {}};
+
+    // Limit the number of XPath expressions for performance reasons. See
+    // bug 477564.
+    const MAX_TRAVERSED_XPATHS = 100;
+    let generatedCount = 0;
+
+    while (node = formNodes.iterateNext()) {
+      let nId = node.id;
+      let hasDefaultValue = true;
+      let value;
+
+      // Only generate a limited number of XPath expressions for perf reasons
+      // (cf. bug 477564)
+      if (!nId && generatedCount > MAX_TRAVERSED_XPATHS) {
+        continue;
+      }
+
+      if (node instanceof Ci.nsIDOMHTMLInputElement ||
+          node instanceof Ci.nsIDOMHTMLTextAreaElement) {
+        switch (node.type) {
+          case "checkbox":
+          case "radio":
+            value = node.checked;
+            hasDefaultValue = value == node.defaultChecked;
+            break;
+          case "file":
+            value = { type: "file", fileList: node.mozGetFileNameArray() };
+            hasDefaultValue = !value.fileList.length;
+            break;
+          default: // text, textarea
+            value = node.value;
+            hasDefaultValue = value == node.defaultValue;
+            break;
+        }
+      } else if (!node.multiple) {
+        // <select>s without the multiple attribute are hard to determine the
+        // default value, so assume we don't have the default.
+        hasDefaultValue = false;
+        value = { selectedIndex: node.selectedIndex, value: node.value };
+      } else {
+        // <select>s with the multiple attribute are easier to determine the
+        // default value since each <option> has a defaultSelected
+        let options = Array.map(node.options, function(aOpt, aIx) {
+          let oSelected = aOpt.selected;
+          hasDefaultValue = hasDefaultValue && (oSelected == aOpt.defaultSelected);
+          return oSelected ? aOpt.value : -1;
+        });
+        value = options.filter(function(aIx) aIx !== -1);
+      }
+
+      // In order to reduce XPath generation (which is slow), we only save data
+      // for form fields that have been changed. (cf. bug 537289)
+      if (!hasDefaultValue) {
+        if (nId) {
+          ret.id[nId] = value;
+        } else {
+          generatedCount++;
+          ret.xpath[XPathGenerator.generate(node)] = value;
+        }
+      }
+    }
+
+    return ret;
+  },
+
+  /**
+   * Merges form data on a document from previously obtained data.
+   *
+   * This is the inverse of getFormData(). The data argument is the same object
+   * type which is returned by getFormData(): an object containing the keys
+   * "id" and "xpath" which are each objects mapping element identifiers to
+   * form values.
+   *
+   * Where the document has existing form data for an element, the value
+   * will be replaced. Where the document has a form element but no matching
+   * data in the passed object, the element is untouched.
+   *
+   * @param  aDocument
+   *         DOMDocument instance to which to restore form data.
+   * @param  aData
+   *         Object defining form data.
+   */
+  mergeFormData: function DocumentUtils_mergeFormData(aDocument, aData) {
+    if ("xpath" in aData) {
+      for each (let [xpath, value] in Iterator(aData.xpath)) {
+        let node = XPathGenerator.resolve(aDocument, xpath);
+
+        if (node) {
+          this.restoreFormValue(node, value, aDocument);
+        }
+      }
+    }
+
+    if ("id" in aData) {
+      for each (let [id, value] in Iterator(aData.id)) {
+        let node = aDocument.getElementById(id);
+
+        if (node) {
+          this.restoreFormValue(node, value, aDocument);
+        }
+      }
+    }
+  },
+
+  /**
+   * Low-level function to restore a form value to a DOMNode.
+   *
+   * If you want a higher-level interface, see mergeFormData().
+   *
+   * When the value is changed, the function will fire the appropriate DOM
+   * events.
+   *
+   * @param  aNode
+   *         DOMNode to set form value on.
+   * @param  aValue
+   *         Value to set form element to.
+   * @param  aDocument [optional]
+   *         DOMDocument node belongs to. If not defined, node.ownerDocument
+   *         is used.
+   */
+  restoreFormValue: function DocumentUtils_restoreFormValue(aNode, aValue, aDocument) {
+    aDocument = aDocument || aNode.ownerDocument;
+
+    let eventType;
+
+    if (typeof aValue == "string" && aNode.type != "file") {
+      // Don't dispatch an input event if there is no change.
+      if (aNode.value == aValue) {
+        return;
+      }
+
+      aNode.value = aValue;
+      eventType = "input";
+    } else if (typeof aValue == "boolean") {
+      // Don't dispatch a change event for no change.
+      if (aNode.checked == aValue) {
+        return;
+      }
+
+      aNode.checked = aValue;
+      eventType = "change";
+    } else if (typeof aValue == "number") {
+      // handle select backwards compatibility, example { "#id" : index }
+      // We saved the value blindly since selects take more work to determine
+      // default values. So now we should check to avoid unnecessary events.
+      if (aNode.selectedIndex == aValue) {
+        return;
+      }
+
+      if (aValue < aNode.options.length) {
+        aNode.selectedIndex = aValue;
+        eventType = "change";
+      }
+    } else if (aValue && aValue.selectedIndex >= 0 && aValue.value) {
+      // handle select new format
+
+      // Don't dispatch a change event for no change
+      if (aNode.options[aNode.selectedIndex].value == aValue.value) {
+        return;
+      }
+
+      // find first option with matching aValue if possible
+      for (let i = 0; i < aNode.options.length; i++) {
+        if (aNode.options[i].value == aValue.value) {
+          aNode.selectedIndex = i;
+          break;
+        }
+      }
+      eventType = "change";
+    } else if (aValue && aValue.fileList && aValue.type == "file" &&
+      aNode.type == "file") {
+      aNode.mozSetFileNameArray(aValue.fileList, aValue.fileList.length);
+      eventType = "input";
+    } else if (aValue && typeof aValue.indexOf == "function" && aNode.options) {
+      Array.forEach(aNode.options, function(opt, index) {
+        // don't worry about malformed options with same values
+        opt.selected = aValue.indexOf(opt.value) > -1;
+
+        // Only fire the event here if this wasn't selected by default
+        if (!opt.defaultSelected) {
+          eventType = "change";
+        }
+      });
+    }
+
+    // Fire events for this node if applicable
+    if (eventType) {
+      let event = aDocument.createEvent("UIEvents");
+      event.initUIEvent(eventType, true, true, aDocument.defaultView, 0);
+      aNode.dispatchEvent(event);
+    }
+  }
+};