Firefox is following the real spec and does not decode '+' to space...in fact it further encodes them to '%2B' to store the cookie. If you read a cookie using javascript and unescape it, all your spaces will be turned to '+'.
To fix this problem, use setrawcookie and rawurlencode:
<?php
setrawcookie('cookie_name', rawurlencode($value), time()+60*60*24*365);
?>
The only change is that spaces will be encoded to '%20' instead of '+' and will now decode properly.