Good job at not reading the article! Had you read it, you would've learned that the commands work like this:

    Find-Package Firefox
    Install-Package Firefox

I admit it's not as easy as on e.g. Debian:

    sudo apt-get remove iceweasel
    sudo echo deb http://packages.linuxmint.com debian import >>/etc/apt/sources.list
    sudo gpg --keyserver pgp.mit.edu --recv-keys 3EE67F3D0FF405B2
    sudo gpg --export 3EE67F3D0FF405B2 > 3EE67F3D0FF405B2.gpg
    sudo apt-key add ./3EE67F3D0FF405B2.gpg
    sudo rm ./3EE67F3D0FF405B2.gpg
    sudo apt-get update
    sudo apt-get install firefox