As of last nightly (2015-07-23), Firefox for Linux is using Gtk+3 instead of Gtk+2.

Thanks to the recent efforts of Andrew Comminos, all remaining test failures are gone and mozilla-central now defaults to Gtk+3 builds. Some jobs on treeherder are still not converted, but this will come soon (bug 1186748).

If you’ve been using elm builds for dogfooding, you should be automatically switched to standard nightlies today or tomorrow. The elm branch will be recycled to do Gtk+2 builds so that they keep working. Those builds won’t be auto-updating, so don’t use them.

http://glandium.org/blog/?p=3573