it works only when 'first' or 'last' are set to an integer value.
i.e.:
echo $this->Paginator->numbers(
array('modulus' => 8,
'separator' => false,
'before' => '',
'after' => '',
'tag' => 'li',
'class' => false,
'currentClass' => 'disabled',
'currentTag' => 'a',
'first' => 3,
'last' => 3,
)
);