The GTK3 port of Firefox is being done for theme compatibility with modern distributions, new tool-kit support, Wayland compatibility, and other advantages. GTK3 on Firefox has been in development for a while and more details can be learned from the Mozilla.org Wiki. Using GTK3 for Firefox will also allow drawing in the title bar for new Firefox themes, Wayland support without X11 dependence, support for multi-touch, greater Cairo usage, and support for new UI widgets.
A lot of the necessary work to support GTK3 in Firefox over GTK2 has been completed since work began around three years ago. Among the work left is changing the plugin-container for GTK3, crash fixes due to GTK3 interface changes, tab widget rendering support, and other changes. The blocker bug for going from GTK2 to GTK3 in Firefox is Bug 627699. Also blocking GTK3 Firefox is NPAPI plug-in support, which lacking Flash support this browser is a non-starter for many, but eventually there will be Mozilla Shumway for Firefox Flash support.
Announced today on the Fedora devel list is a "tech preview" source RPM of Firefox built against GTK3 for the Linux distribution.
Mozilla probably won't release an official GTK3 version of Firefox ahead of the Fedora 21 release this summer, but at least progress on the Firefox GTK3 port is continuing and maybe we'll see something before the end of 2014.