Can the AppCenter do it just like how Ubuntu Software does?
It seems like AppCenter lacks lots of apps.
Firefox is already available in AppCenter.
AppCenter doesn't support the installation of Snap packages at this time
i think if you want you can, since loki is built on ubuntu 16.04 first install snapd
sudo apt install snapd
then install snap applications normally.
For example to install a simple app just run
sudo snap install hello to see the installed app use
snap list to run an installed app use
snap run hello
if you need to know more how to manage and install snap apps refer this link https://goo.gl/aFfwvM