You will need to install a user script manager extension to install this script.
Try click on this link: https://i.pximg.net/img-original/img/2013/01/09/00/50/59/32740714p0.jpg?preventcache
You will see 403 Forbidden if you doesn't have this script. The error will no longer exists after installing this script.
If you want to get full resolution image, you could install Pixiv Replace With Original.