Set the following preferences
This problem seems to plague most Thunderbird users on Mac OS X – myself included. So I have been scouring the interwebs looking for a solution and today I’ve found one. The default behavior for Thunderbird is to use the Desktop when “opening” attachments but there is a way to change this. There’s a preference which determines how Thunderbird chooses the “temporary download” directory.
The acceptable values for this are 0, 1, or 2 which respectively correspond to the Desktop, the Downloads, or a Custom folder. So if we set this preference to 2, then Thunderbird will try and use the folder that is defined in the preference
Setting these two preferences will finally stop Thunderbird from cluttering you desktop with attachemtns on Mac OS X.
-The IT Department