Application not responding
From MozillaZine Knowledge Base
Thunderbird stops responding and may use large amounts of processing power.
In some cases, the problem can be caused by a mail summary file (*.msf) file being corrupted. This can occur when your folder is mildly corrupted due to not compacting it regularly. A quick and dirty fix is to:
If that doesn't work, it may be because your folder is too badly corrupted. See the second part of the "Compacting doesn't seem to work" section in Compacting folders.
If the problem occurs when sending a message, try re-configuring your anti-virus program so it does not check outgoing messages for viruses.
In some cases, the problem is caused when Thunderbird has trouble detecting and filtering junk (spam). This may be caused by a corrupted training.dat file. It may be necessary to delete this (Tools -> Options -> Privacy -> Junk -> Reset Training Data -> OK) and retrain the [Mail Controls].