Comment 384 for bug 191889

For me, I don't think it's FF3 problem. It's network-manager's problem. I could get connected to internet through PPPoE and browse normally with FF3, but network-manager applet failed to detect it as connected.

Solution (maybe temporary) with Ubuntu Intrepid 8.10:
1. remove network-manager
sudo apt-get remove network manager

2. Install wicd
http://www.ubuntugeek.com/wicd-wired-and-wireless-network-manager-for-ubuntu.html

3. When asked about wicd pubkey, just
wget -q http://apt.wicd.net/wicd.gpg -O- | sudo apt-key add -

Restart your Ubuntu as although you have remove network-manager, it can give you some error message.

Good luck!