A "crash" is what happens when Thunderbird closes unexpectedly. After a crash, the Mozilla Crash Reporter TB should appear, asking if you want to send crash information to Mozilla for processing.
For information about crashes and hangs caused by plug-ins, see Plugin Crash Reports TB.
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Getting help with your crash
There are many possible causes for crashes, so the best way to get good help is to post your crash report ID or attach a stacktrace in a forum or bug report. These will help to identify the cause of crash.
Crash reports are given an ID of the form BP-nnnnnn. See Viewing crash reports for how to get your crash report ID. This ID is linked in the crash database and provides our developers and support helpers with information about what happened in the crash.
If crash reporter didn't appear, or failed to submit its crash report, please see How to get a stacktrace for a bug report, and substitute the string Thunderbird wherever you see Firefox in the instructions.
General Crash Help and Suggestions
Not using the latest version of Thunderbird
Thunderbird updates are regularly released and a number of the fixes are crash related. Make sure you are using the latest version. See Updating Thunderbird for details.
Thunderbird crashes can be caused by Add-ons. Does Thunderbird crash when started in Safe Mode TB?