From MozillaZine Knowledge Base

Symptoms

In Windows, Thunderbird is not responding. It may also be using 100% of the processing power.

Solution

In some cases, this can be caused by a mail summary file (*.msf) file being corrupted. This can occur when your folder is mildy corrupted due to not compacting it regularly. A quick and dirty fix is to:

1. Exit Thunderbird.
2. Find your profile folder.
3. Open the folder containing your stored messages. For example, if you are using the Global Inbox, this would be the "Local Folders" folder inside the "Mail" folder. Otherwise its a folder named after the mail server, inside the "Mail" folder. For example, "pop.gmail.com" if you're using gmail.
4. Select the *.msf files and delete it.
5. Restart Thunderbird.
6. Wait for Thunderbird to recreate the mail summary file(s).

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 not to check outgoing messages for viruses.

See also

• Keep it working (Thunderbird)
• Standard diagnostic for Thunderbird

External link

• Bug 243701: Thunderbird freezes / crashes after inactivity