How to prevent Firefox from updating to version 57 on Ubuntu 16.04?

Simply do:

sudo apt-mark hold firefox

This will add firefox package to the list of packages who should not recieve updates.

To reverse it:

sudo apt-mark unhold firefox

You can list the packages on hold via:

sudo apt-mark showhold

For further information see man apt-mark.8

I think you've installed firefox via apt, so:

echo firefox hold | sudo dpkg --set-selections