captcha image doesn't appear in FF , why? :(

[sasori]

why is it that the captcha image doesn't appear in firefox ? but it does appear in chrome, is there something wrong with this code ? 

	//create captcha for verification
	$captcha = new Zend_Form_Element_Captcha('Captcha', array(
				'captcha' => array(
					'captcha' => 'Image',
					'wordLen' => 6,
					'timeout' => 300,
					'width' => 300,
					'height' => 100,
					'imgUrl' => 'public/captcha',
					'imgDir' => APPLICATION_PATH .'/../public/captcha',
					'font' => APPLICATION_PATH .'/../public/fonts/LiberationSansRegular.ttf'
					)
	));