GtkMenuItem's "label" property was introduced in GTK+ 2.16, according to 
<https://bugreports.qt-project.org/browse/QTBUG-8537>. You're using GTK+ 
2.10.4, which could explain the problem. I'm using Firefox 20 which has 
GTK+ 3.10.6 and do not see the problem.

I suspect that the attached patch (which is trunk bzr 115967.1.1) is 
what introduced the problem. If you back this change out, does it fix 
things? I'll CC: this to Daniel Colascione to give him a heads-up.