If a debian based distro there's several ways to copy your package choices from one machine to another, the likes of using dpkg --get-selections and using that output on the new machine with --set-selections, but probably the easiest way is to use apt-clone, which streamlines that process to install the same packages on the new machine.

As for your firefox and desktop packages, they'll be saved in /home/username (usually in .hidden dirs) - so just copy all of that over.

Alternatively, use an image cloner like clonezilla to make an exact disk copy and install that.

Or run your machine as a vm inside Proxmox or another hypervisor. That way you can have instant snapshots before you do risky things, as well as multiple scheduled entire-machine backups.

Or ansible (which I do a lot of), which can set whatever packages and copy your golden-image files over as you want. (But keeping those up to date requires a little thought)