You need to substitute all lines with address containing ports.ubuntu.com.
If you want to use e.g. http://ftp.tu-chemnitz.de/pub/linux/ubuntu-ports/
Your file will be:
deb http://ftp.tu-chemnitz.de/pub/linux/ubuntu-ports/ xenial main universe multiverse restricted
deb-src http://ftp.tu-chemnitz.de/pub/linux/ubuntu-ports/ xenial main universe multiverse restricted
deb http://ftp.tu-chemnitz.de/pub/linux/ubuntu-ports/ xenial-security main universe multiverse restricted
deb-src http://ftp.tu-chemnitz.de/pub/linux/ubuntu-ports/ xenial-security main universe multiverse restricted
deb http://ftp.tu-chemnitz.de/pub/linux/ubuntu-ports/ xenial-updates main universe multiverse restricted
deb-src http://ftp.tu-chemnitz.de/pub/linux/ubuntu-ports/ xenial-updates main universe multiverse restricted
deb https://files.husarion.com/repo xenial main ros rpi backports
deb http://packages.ros.org/ros/ubuntu xenial main
deb http://ppa.launchpad.net/mozillateam/ppa/ubuntu xenial main
Regards,
Łukasz