I see this both with firefox -app on OpenBSD and xulrunner on Arch Linux. Nothing has change in my X or
conkeror configuration recently, and the meta key continues to work in emacs and xterm.

$ uname -a
OpenBSD jet 5.2 GENERIC.MP#326 amd64
$ firefox --version
Mozilla Firefox 13.0.1

$ uname -a
Linux shale 3.3.8-1-ARCH #1 SMP PREEMPT Tue Jun 5 15:20:32 CEST 2012 x86_64 GNU/Linux
$ xulrunner --version
Mozilla XULRunner 13.0 - 20120606114039