This writes a 1-line firefox wrapper script into /usr/local/bin (so it takes priority over the regular firefox command at /usr/bin/firefox).

The script loads firefox with the option of a set profile directory on PorteuX's liveUSB medium (so it's persistent)

/mnt/sda1/P/firefox.default

To change this it's necessary to unpack the .xzm and manually edit the script, then repack it again.