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($value), time()+60*60*24*365);
?>
The only change is that spaces will be encoded to '%20' instead of '+' and will now decode properly.setrawcookie() isn't entirely 'raw'. It will check the value for invalid characters, and then disallow the cookie if there are any. These are the invalid characters to keep in mind: ',;<space>\t\r\n\013\014'.
Note that comma, space and tab are three of the invalid characters. IE, Firefox and Opera work fine with these characters, and PHP reads cookies containing them fine as well. However, if you want to use these characters in cookies that you set from php, you need to use header().You really shouldn't use (un)serialize with cookies. An evil user could inject ANY code in your script.If you want to pass something and unserialize later, you should somehow sign value to ensure evil user don't modify it.
For example, calculate hash sha1($value.$securekey) and place it to different cookie. If cookie value mismatch hash - simple discard both.
This technique you can use in any case if you want to protect cookie from modification, but it can't protect from deletion or from setting to other valid cookie (old or stolen from other user).