The nsMsgViewSortOrder interface contains constants used for sort direction in Thunderbird. This interface isn't ever implemented. It is only used to store constants. For example to sort by date you would pass a function the value: 

 Components.interfaces.nsMsgViewSortOrder.ascending


mailnews/base/public/nsIMsgDBView.idlScriptable
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  Last changed in Gecko 1.9 (Firefox 3)

Constants

Name Value Description
none 0
ascending 1
descending 2