Index: includes/ajax.inc
===================================================================
RCS file: /cvs/drupal/drupal/includes/ajax.inc,v
retrieving revision 1.41
diff -u -p -r1.41 ajax.inc
--- includes/ajax.inc	23 Dec 2010 04:26:31 -0000	1.41
+++ includes/ajax.inc	26 Dec 2010 22:02:10 -0000
@@ -587,13 +587,8 @@ function ajax_pre_render_element($elemen
       case 'submit':
       case 'button':
       case 'image_button':
-        // Use the mousedown instead of the click event because form
-        // submission via pressing the enter key triggers a click event on
-        // submit inputs, inappropriately triggering AJAX behaviors.
-        $element['#ajax']['event'] = 'mousedown';
-        // Attach an additional event handler so that AJAX behaviors
-        // can be triggered still via keyboard input.
-        $element['#ajax']['keypress'] = TRUE;
+      case 'link':
+        $element['#ajax']['event'] = 'click';
         break;
 
       case 'password':
@@ -608,10 +603,6 @@ function ajax_pre_render_element($elemen
         $element['#ajax']['event'] = 'change';
         break;
 
-      case 'link':
-        $element['#ajax']['event'] = 'click';
-        break;
-
       default:
         return $element;
     }
Index: misc/ajax.js
===================================================================
RCS file: /cvs/drupal/drupal/misc/ajax.js,v
retrieving revision 1.35
diff -u -p -r1.35 ajax.js
--- misc/ajax.js	23 Dec 2010 04:26:31 -0000	1.35
+++ misc/ajax.js	26 Dec 2010 22:02:10 -0000
@@ -55,7 +55,7 @@ Drupal.behaviors.AJAX = {
 
     // This class means to submit the form to the action using AJAX.
     $('.use-ajax-submit:not(.ajax-processed)').addClass('ajax-processed').each(function () {
-      var element_settings = {};
+      var element_settings = {event: 'click'};
 
       // AJAX submits specified in this manner automatically submit to the
       // normal form action.
@@ -63,14 +63,33 @@ Drupal.behaviors.AJAX = {
       // Form submit button clicks need to tell the form what was clicked so
       // it gets passed in the POST request.
       element_settings.setClick = true;
-      // Form buttons use the 'click' event rather than mousedown.
-      element_settings.event = 'click';
       // Clicked form buttons look better with the throbber than the progress bar.
-      element_settings.progress = { 'type': 'throbber' };
+      element_settings.progress = {'type': 'throbber'};
 
       var base = $(this).attr('id');
       Drupal.ajax[base] = new Drupal.ajax(base, this, element_settings);
     });
+
+    // Some browsers (e.g., Firefox) trigger the click event of the first submit
+    // button when ENTER is pressed from a textfield. Some browsers (e.g., IE7)
+    // trigger form submission directly rather than triggering a button click,
+    // but the Form API processes this as though the first button were clicked.
+    // Some browsers invoke this implicit submission behavior only when a single
+    // textfield exists on the form; others invoke it regardless of how many
+    // textfields are on the form. In any case, when AJAX is bound to the first
+    // submit button, disable this browser behavior, because it is unlikely that
+    // the user intended to trigger the button's AJAX behavior when pressing
+    // ENTER from a textfield.
+    // @see http://drupal.org/node/634616
+    $('input:text', context).once('ajax-prevent-implicit-activation', function() {
+      // Namespace the event so that modules may unbind it and replace with an
+      // alternate implementation.
+      $(this).bind('keypress.ajaxPreventImplicitActivation', function(event) {
+        if (event.which == 13 && $('input:submit, input:image', this.form).filter(':eq(0)').filter('.ajax-processed').length) {
+          event.preventDefault();
+        }
+      });
+    });
   }
 };
 
@@ -99,8 +118,8 @@ Drupal.behaviors.AJAX = {
 Drupal.ajax = function (base, element, element_settings) {
   var defaults = {
     url: 'system/ajax',
-    event: 'mousedown',
-    keypress: true,
+    event: 'click',
+    keypress: false,
     selector: '#' + base,
     effect: 'none',
     speed: 'none',
Index: modules/field_ui/field_ui.js
===================================================================
RCS file: /cvs/drupal/drupal/modules/field_ui/field_ui.js,v
retrieving revision 1.8
diff -u -p -r1.8 field_ui.js
--- modules/field_ui/field_ui.js	21 Nov 2010 08:50:49 -0000	1.8
+++ modules/field_ui/field_ui.js	26 Dec 2010 22:03:16 -0000
@@ -236,7 +236,7 @@ Drupal.fieldUIOverview = {
 
       // Fire the AJAX update.
       $('input[name=refresh_rows]').val(rowNames.join(' '));
-      $('input#edit-refresh').mousedown();
+      $('input#edit-refresh').click();
 
       // Disabled elements do not appear in POST ajax data, so we mark the
       // elements disabled only after firing the request.
Index: modules/file/file.js
===================================================================
RCS file: /cvs/drupal/drupal/modules/file/file.js,v
retrieving revision 1.6
diff -u -p -r1.6 file.js
--- modules/file/file.js	23 Nov 2010 05:51:16 -0000	1.6
+++ modules/file/file.js	26 Dec 2010 22:03:16 -0000
@@ -37,12 +37,12 @@ Drupal.behaviors.fileValidateAutoAttach 
  */
 Drupal.behaviors.fileButtons = {
   attach: function (context) {
-    $('input.form-submit', context).bind('mousedown', Drupal.file.disableFields);
-    $('div.form-managed-file input.form-submit', context).bind('mousedown', Drupal.file.progressBar);
+    $('input.form-submit', context).bind('click', Drupal.file.disableFields);
+    $('div.form-managed-file input.form-submit', context).bind('click', Drupal.file.progressBar);
   },
   detach: function (context) {
-    $('input.form-submit', context).unbind('mousedown', Drupal.file.disableFields);
-    $('div.form-managed-file input.form-submit', context).unbind('mousedown', Drupal.file.progressBar);
+    $('input.form-submit', context).unbind('click', Drupal.file.disableFields);
+    $('div.form-managed-file input.form-submit', context).unbind('click', Drupal.file.progressBar);
   }
 };
 
@@ -105,10 +105,11 @@ Drupal.file = Drupal.file || {
     // working with. Filter out fields that are already disabled so that they
     // do not get enabled when we re-enable these fields at the end of behavior
     // processing. Re-enable in a setTimeout set to a relatively short amount
-    // of time (1 second). All the other mousedown handlers (like Drupal's AJAX
-    // behaviors) are excuted before any timeout functions are called, so we
-    // don't have to worry about the fields being re-enabled too soon.
-    // @todo If the previous sentence is true, why not set the timeout to 0?
+    // of time (1 second). For AJAX form submissions involving file uploads, the
+    // browser's native form submission (to an IFRAME), rather than JavaScript
+    // serialization to an XHR object, is used, so the fields must be disabled
+    // when the browser initiates the POST request. 1 second should be long
+    // enough.
     var $fieldsToTemporarilyDisable = $('div.form-managed-file input.form-file').not($enabledFields).not(':disabled');
     $fieldsToTemporarilyDisable.attr('disabled', 'disabled');
     setTimeout(function (){