I've disabled the Mozilla Pie Menus [1]. While a neat idea [2] the current implementation proved to be too unstable to use, especially when you hit a fairly heavy-hitting CSS (Cascading Style Sheets)-based site (like, oh, this one? [3]).
Well, it wasn't that I uninstalled the pie menus as I reinstalled Mozilla [4] (well, installed the latest version).
At least I have the Site Navigation Bar back.
[1] http://optimoz.mozdev.org/piemenus/