Comment 3 for bug 1698903

That's because the current artful package is an older build version of Firefox (50.1.0 - https://packages.ubuntu.com/artful/firefox) that doesn't have them. Compact themes were introduced in 53.0 (https://www.mozilla.org/firefox/53.0/releasenotes/).

Why they haven't updated the package yet I don't know, they probably will once artful reaches beta.