Yes, it’s the same Firefox using which I am writing this article. Firefox nightlies for Linux from Jul 23, 2015 are compiled with Gtk+3. Firefox 42 can be expected to have Gtk+3 on release.
There are some glitches around showing borders of address and search bars but that’s about it. No crashes even after hours of use. Preferences, about:config switches, add-ons (at least those I use) work as expected. As per the official notes some work still remains to be converted to Gtk+3.
I must mention that I am noticing a huge boost in speed, both with start-up and fetching webpages.
Download the latest nightly for your architecture and give it a spin!