there's available  new Firefox package with emulated CSD rendering .
What does it mean for you? Firefox renders CSD decorations on its own
then and you don't see the default gnome titlebar on top of the main
window and you get a "Chromish" look. That should also work with Firefox
But you need to enable it explicitly at about:config. Set
"widget.allow-client-side-decoration" to true and restart the browser.
Please report any issues to bugzilla.redhat.com
and mentions it's the
CSD version. I'm also interested on testing when
"widget.allow-client-side-decoration" is false - to make sure it does
not break anything.
Fedora 25: https://koji.fedoraproject.org/koji/buildinfo?buildID=970606
Fedora 26: https://koji.fedoraproject.org/koji/buildinfo?buildID=970604
Fedora 27: https://koji.fedoraproject.org/koji/buildinfo?buildID=970605