Set focus on Extjs textfield issue

Sometimes it helps to add a little delay when focusing. This is because certain browsers (Firefox 3.6 for sure) need a bit of extra time to render form elements.
Note that ExtJS's focus() method accepts defer as an argument, so try that:

Ext.getCmp('first_name').focus(false, 200);

 OR 

    fieldLabel: 'First Name',
    name: 'first',
    id: 'first_name',
    allowBlank: false,
    listeners: {
      afterrender: function(field) {
        field.focus(false, 1000);
      }
    }