I was able to sort this by adding some configuration options to the field,
Add the following code above addField () of your WYSIWYG,
$configSettings = Mage::getSingleton('cms/wysiwyg_config')->getConfig( array( 'add_widgets' => false, 'add_variables' => false, 'add_images' => false, 'files_browser_window_url'=> $this->getBaseUrl().'admin/cms_wysiwyg_images/index/'));
After you have added the code, you need to add another parameter to addField, called 'config', which calls the $ configSettings variable.
$fieldset->addField('post_content', 'editor', array( 'name' => 'post_content', 'label' => Mage::helper('faqs')->__('Answer'), 'title' => Mage::helper('faqs')->__('Answer'), 'style' => 'width:700px; height:500px;', 'wysiwyg' => true, 'config' => $configSettings ));
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