How to add WYSIWYG functionality to Magento Config items

First of all add this in any layout file, to load the editor in the config section:

<adminhtml_system_config_edit>
    <update handle="editor"/>
    <reference name="head">
        <action method="setCanLoadTinyMce"><load>1</load></action>
    </reference>
</adminhtml_system_config_edit>

Now create your own field renderer. It has to be a block inside your module:

<?php
class Namespace_Module_Block_Adminhtml_System_Config_Editor extends 
Mage_Adminhtml_Block_System_Config_Form_Field 
implements Varien_Data_Form_Element_Renderer_Interface{
    protected function _getElementHtml(Varien_Data_Form_Element_Abstract $element){
        $element->setWysiwyg(true);
        $element->setConfig(Mage::getSingleton('cms/wysiwyg_config')->getConfig());
        return parent::_getElementHtml($element);
    }
}

Now for the element inside the system.xml set the frontend_type ‘editor’ and the frontend_model your new block

<fieldname translate="label">
    <label>Field label </label>
    <frontend_type>editor</frontend_type>
    <frontend_model>module/adminhtml_system_config_editor</frontend_model>
    <sort_order>150</sort_order>
    <show_in_default>1</show_in_default>
    <show_in_website>1</show_in_website>
    <show_in_store>1</show_in_store>
</fieldname>