Firefox GTK3 builds are still pre-nightly. We won't uplift such major work, current nightly is 34 with the uplift to 35 on the day after labor day. I would say that 36 would be possible but obviously depends on the work load of Glandium and Emilio Pozuelo.

If you want to test the builds the are available at ftp://ftp.mozilla.org/pub/firefox/tinderbox-builds/elm-linux64/ however there is no updater, so you would need to manually do that.