Error uploading file pdf

Sorry, I do not speak English, but I have a question.
I need to upload one file only with extension pdf.

My problem is only using browser firefox, i not have problem using chrome.

My code is

Code:

public function uploadFile()
{
$config['upload_path'] = dirname($_SERVER["SCRIPT_FILENAME"])."/upload/";
$config['upload_url'] = base_url()."upload/";
$config['allowed_types'] = "pdf";
$config['overwrite'] = TRUE;
$config['max_size'] = "1000KB";
$config['file_name'] = "namearchivo";
$this->load->library('upload', $config);

if($this->upload->do_upload())
{
$file_name = $_FILES['userfile']['name'];
redirect($urlAnterior);
} else {
echo "No ok";
redirect($urlAnterior);
}
}