(This was answered on the devel list here
Firefox 2.0 came out too close to the FC6 release to be tested properly. Many things depend on Firefox so it's important for it to be stable. It's not planned for FC6 updates either; the main Moz.org person at RH said basically it was too big a change with too few benefits to make before FC7.
But FC7 is only a few months away....