Found a work around that prevents Firefox 3 from enabling work offline when you are using a USB modem or in my case an iBurst pcmcia modem.
The fix originally posted here requires you to open the Network Manager configuration file.
$ gksudo gedit /etc/dbus-1/system.d/NetworkManager.conf
Then replace <allow send_interface=”org.freedesktop.NetworkManager”/> with <deny send_interface=”org.freedesktop.NetworkManager”/>.
After saving the changes, restart your machine.
What about ‘Toolkit.networkmanager.disable’ set to true? This will also do the trick, much faster and easier
(about:config) in FF.