Guys,
there's available [1] new Firefox package with emulated CSD rendering [2].
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 light themes.
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.
Thanks! ma.
[1] 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