Comment 2 for bug 550922

I'm sorry to hear Ubuntu One is not working properly for you. If ubuntuone is not launching firefox to add your computer after deleting that computer, you may still have the ubuntuone "token" in the keyring (seahorse).

Please do the following to delete the ubuntuone token:
1 Open Applications -> Accessories -> Password and Encryption Keys
2 Click on the arrow next to "Passwords:default
3 Right-click on the Ubuntu One token and select "Delete"
4 Left-click on the ubuntuone applet & select "connect". Firefox should now open & ask you to add your computer.