Comment 6 for bug 1278942

This was fixed in Trusty apparently, accessibility for Firefox and VirtualBox is back for me. Apparently Ubuntu replaced gnome-settings-daemon with its own unity-settings-daemon, and the new one initially failed to locate the GTK modules for the various at-spi bridges, i.e. for Mozilla apps, Qt, and probably Java as well.

This won't have any direct effect on Ubuntu 13.10, but perhaps something similar is going on. Lee, when you find the time run this please (while Firefox is running):

$ grep -i atk-bridge /proc/$(pidof firefox)/maps

It should print lines like
/usr/lib/x86_64-linux-gnu/gtk-2.0/modules/libatk-bridge.so

and

$ env | grep GTK_MODULES

which here is
GTK_MODULES=gail:atk-bridge:overlay-scrollbar:unity-gtk-module