Ubuntu is by standard set up for unattended upgrades for security patches and updates. Since Firefox 57 was released through the security repositories you might feel it is abusing root rights, but in fact it isn't, it just got installed through unattended upgrades.
$ apt policy firefox
*** 57.0.1+build2-0ubuntu0.17.10.1 500
500 http://archive.ubuntu.com/ubuntu artful-updates/main amd64 Packages
500 http://archive.ubuntu.com/ubuntu artful-security/main amd64 Packages
500 http://archive.ubuntu.com/ubuntu artful/main amd64 Packages
If you want to verify that you simply can have a look at
/var/log/apt/history.log and search for the updates that happened recently. On its own Firefox on Linux is unable to update itself like it does on Windows.
You could have put Firefox on hold beforehand but now as you have been upgraded already to the newest version you should look into ways to downgrade it or alternatives