After Fedora, Firefox-GTK3 is now available for Arch-ers, through AURepository, thanks to “Yosef Or Boczko” (GNOME Contributor).
Pay attention to the the dark inputs, when we are using Adwaita Dark Theme. That’s a new bug, because the first build of Firefox GTK3 hadn’t that issue. Notice that Firefox GTK3 port, is still under development and not ready for everyday use.
Installing should be as simple as
$ yaourt -S firefox-gtk3-bin
Remember that you can have both Firefox versions installed simultaneously. Some more information and links about the GTK3 port in Firefox can be found in an older post.
Don’t miss to file bugs!
Update: The package moved from the name firefox-gtk3 to firefox-gtk3-bin