diff --git a/lodash.src.js b/lodash.src.js
index 86a2a6036..b44470634 100644
--- a/lodash.src.js
+++ b/lodash.src.js
@@ -772,6 +772,7 @@
         ceil = Math.ceil,
         clearTimeout = context.clearTimeout,
         floor = Math.floor,
+        getOwnPropertySymbols = isNative(getOwnPropertySymbols = Object.getOwnPropertySymbols) && getOwnPropertySymbols,
         getPrototypeOf = isNative(getPrototypeOf = Object.getPrototypeOf) && getPrototypeOf,
         push = arrayProto.push,
         propertyIsEnumerable = objectProto.propertyIsEnumerable,
@@ -793,6 +794,18 @@
       return result;
     }());
 
+    /** Used as `baseAssign`. */
+    var nativeAssign = (function() {
+      var object = Object('x'),
+          func = isNative(func = Object.assign) && func;
+
+      // Avoid `Object.assign` in Firefox 34-37 which have an early implementation
+      // with a slower try/catch behavior. See https://bugzilla.mozilla.org/show_bug.cgi?id=1103344
+      // for more details.
+      try { func(object, 'xo'); } catch(e) {}
+      return !object[1] && func;
+    }());
+
     /* Native method references for those with the same name as other `lodash` methods. */
     var nativeIsArray = isNative(nativeIsArray = Array.isArray) && nativeIsArray,
         nativeCreate = isNative(nativeCreate = Object.create) && nativeCreate,
@@ -1747,21 +1760,19 @@
     }
 
     /**
-     * The base implementation of `_.assign` without support for argument juggling,
-     * multiple sources, and `this` binding `customizer` functions.
+     * A specialized version of `_.assign` for customizing assigned values without
+     * support for argument juggling, multiple sources, and `this` binding `customizer`
+     * functions.
      *
      * @private
      * @param {Object} object The destination object.
      * @param {Object} source The source object.
-     * @param {Function} [customizer] The function to customize assigning values.
-     * @returns {Object} Returns the destination object.
+     * @param {Function} customizer The function to customize assigned values.
+     * @returns {Object} Returns `object`.
      */
-    function baseAssign(object, source, customizer) {
-      var props = keys(source);
-      if (!customizer) {
-        return baseCopy(source, object, props);
-      }
+    function assignWith(object, source, customizer) {
       var index = -1,
+          props = keys(source),
           length = props.length;
 
       while (++index < length) {
@@ -1777,6 +1788,21 @@
       return object;
     }
 
+    /**
+     * The base implementation of `_.assign` without support for argument juggling,
+     * multiple sources, and `customizer` functions.
+     *
+     * @private
+     * @param {Object} object The destination object.
+     * @param {Object} source The source object.
+     * @returns {Object} Returns `object`.
+     */
+    var baseAssign = nativeAssign || function(object, source) {
+      return source == null
+        ? object
+        : baseCopy(source, getSymbols(source), baseCopy(source, keys(source), object));
+    };
+
     /**
      * The base implementation of `_.at` without support for string collections
      * and individual key arguments.
@@ -1809,15 +1835,13 @@
      *
      * @private
      * @param {Object} source The object to copy properties from.
-     * @param {Object} [object={}] The object to copy properties to.
      * @param {Array} props The property names to copy.
+     * @param {Object} [object={}] The object to copy properties to.
      * @returns {Object} Returns `object`.
      */
-    function baseCopy(source, object, props) {
-      if (!props) {
-        props = object;
-        object = {};
-      }
+    function baseCopy(source, props, object) {
+      object || (object = {});
+
       var index = -1,
           length = props.length;
 
@@ -1897,7 +1921,7 @@
           }
           result = initCloneObject(isFunc ? {} : value);
           if (!isDeep) {
-            return baseCopy(value, result, keys(value));
+            return baseAssign(result, value);
           }
         } else {
           return cloneableTags[tag]
@@ -2536,7 +2560,7 @@
      * @param {Function} [customizer] The function to customize merging properties.
      * @param {Array} [stackA=[]] Tracks traversed source objects.
      * @param {Array} [stackB=[]] Associates values with source counterparts.
-     * @returns {Object} Returns the destination object.
+     * @returns {Object} Returns `object`.
      */
     function baseMerge(object, source, customizer, stackA, stackB) {
       if (!isObject(object)) {
@@ -4114,6 +4138,15 @@
      */
     var getLength = baseProperty('length');
 
+    /**
+     * Creates an array of the own symbols of `object`.
+     *
+     * @private
+     * @param {Object} object The object to inspect.
+     * @returns {Array} Returns the array of symbols.
+     */
+    var getSymbols = getOwnPropertySymbols || constant([]);
+
     /**
      * Gets the view, applying any `transforms` to the `start` and `end` positions.
      *
@@ -8650,7 +8683,7 @@
      * @category Lang
      * @param {*} value The value to compare.
      * @param {*} other The other value to compare.
-     * @param {Function} [customizer] The function to customize comparing values.
+     * @param {Function} [customizer] The function to customize value comparisons.
      * @param {*} [thisArg] The `this` binding of `customizer`.
      * @returns {boolean} Returns `true` if the values are equivalent, else `false`.
      * @example
@@ -8803,7 +8836,7 @@
      * @category Lang
      * @param {Object} object The object to inspect.
      * @param {Object} source The object of property values to match.
-     * @param {Function} [customizer] The function to customize comparing values.
+     * @param {Function} [customizer] The function to customize value comparisons.
      * @param {*} [thisArg] The `this` binding of `customizer`.
      * @returns {boolean} Returns `true` if `object` is a match, else `false`.
      * @example
@@ -9152,7 +9185,7 @@
      * @category Object
      * @param {Object} object The destination object.
      * @param {...Object} [sources] The source objects.
-     * @param {Function} [customizer] The function to customize assigning values.
+     * @param {Function} [customizer] The function to customize assigned values.
      * @param {*} [thisArg] The `this` binding of `customizer`.
      * @returns {Object} Returns `object`.
      * @example
@@ -9168,7 +9201,11 @@
      * defaults({ 'user': 'barney' }, { 'age': 36 }, { 'user': 'fred' });
      * // => { 'user': 'barney', 'age': 36 }
      */
-    var assign = createAssigner(baseAssign);
+    var assign = createAssigner(function(object, source, customizer) {
+      return customizer
+        ? assignWith(object, source, customizer)
+        : baseAssign(object, source);
+    });
 
     /**
      * Creates an object that inherits from the given `prototype` object. If a
@@ -9209,7 +9246,7 @@
       if (guard && isIterateeCall(prototype, properties, guard)) {
         properties = null;
       }
-      return properties ? baseCopy(properties, result, keys(properties)) : result;
+      return properties ? baseAssign(result, properties) : result;
     }
 
     /**
@@ -9773,7 +9810,7 @@
      * @category Object
      * @param {Object} object The destination object.
      * @param {...Object} [sources] The source objects.
-     * @param {Function} [customizer] The function to customize merging properties.
+     * @param {Function} [customizer] The function to customize assigned values.
      * @param {*} [thisArg] The `this` binding of `customizer`.
      * @returns {Object} Returns `object`.
      * @example
@@ -10745,9 +10782,9 @@
         options = otherOptions = null;
       }
       string = baseToString(string);
-      options = baseAssign(baseAssign({}, otherOptions || options), settings, assignOwnDefaults);
+      options = assignWith(baseAssign({}, otherOptions || options), settings, assignOwnDefaults);
 
-      var imports = baseAssign(baseAssign({}, options.imports), settings.imports, assignOwnDefaults),
+      var imports = assignWith(baseAssign({}, options.imports), settings.imports, assignOwnDefaults),
           importsKeys = keys(imports),
           importsValues = baseValues(imports, importsKeys);