I believe the idea is that there aren't enough Windows people to work on both 64-bit and the Metro UI for Firefox. Metro is a priority, so 64-bit is getting mothballed for the near future.

[1] https://groups.google.com/d/msg/mozilla.dev.apps.firefox/jpX...

That's kind of a shame, because I don't think Metro will be sticking around for very long.

I'm not sure it won't stick around. People said that about Aqua (OS X) and the Gnome 3 shell.

MFC, Windows Forms, WPF, Silverlight ... all of them still around, but all of them being obsolete by whatever Microsoft's du-jour UI framework happens to be.

Gnome 3 is not the same thing, as Gnome 3 is just a window manager, based on the same X, the same APIs and the same GTK, just as Unity and Gnome 2.

bzbarsky posted a longish comment on another subthread: http://news.ycombinator.com/item?id=4838078