A "crash " is what happens when Thunderbird closes unexpectedly. After a crash, the Mozilla Crash Reporter should appear, asking if you want to send the crash information to Mozilla for processing.
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Getting help with your crash
There are many possible causes for crashes, therefore the best way to get efficient help is to post your crash report ID or attach a stacktrace in a forum or a bug report. This will help to identify the cause of your crash.
Crash reports are given an ID of the form BP-nnnnnn. Read Viewing crash reports to learn 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 the Mozilla Crash Reporter didn't appear or failed to submit its crash report, see How to get a stacktrace for a bug report on MDN and substitute Thunderbird wherever you see Firefox in the instructions.
General crash help and suggestions
Make sure to use the latest version of Thunderbird
Thunderbird updates are regularly released and a number of the fixes in them are crash related. Make sure you are using the latest version. See Updating Thunderbird for details.
Check for problematic add-ons
Thunderbird crashes can be caused by add-ons. Does Thunderbird still crash when it is started in Safe Mode? If not, an add-on may contain a bug, or it may not have been updated to be fully compatible with an updated version of Thunderbird. See the Add-ons and Extensions FAQ for details on disabling and updating add-ons.