If you want Firefox 57 in 42.2 just download the tar file from mozilla. You can extract it somewhere on your system (I use /usr/share), create a symlink to the executable in /usr/bin and it works fine.
I assume you know that it drops all your extensions and not all are replaceable.
I installed FF 57 from firefox.com, works fine, including the extensions.
The only thing I don’t like is the filechooser dialog when saving files. I guess there’s no way to change it to the native KDE dialog?
It’s not a showstopper, just annoying.