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.