David,
    I am not going to be much help with the Thunderbird question, but
here is my best guess for Ubuntu updates since you don't list what
version of Ubuntu you have.  Is it possible that you can pick another
download source?  Your connection to the respositories might have to
be reset.  Try the following..

From Update Manager, click on Settings, click on the Ubuntu Software
tab, and select another mirror under "Download From"[select main
server]

hope that helps,
Derek

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