After upgrading to TB3 users on Mac OSX ones finds that a copy of every attachment you open is saved to the Desktop. There is a underlying problem in the Gecko engine causing this, but until then the fix is pretty simple.

• Click Thunderbird > Preferences > Attachments
• Select “Save files to..” click “Choose..”
• Hit COMMAND+SHIFT+G and type in /tmp , Hit GO and then Open
• Switch back the toggle back to “Always Ask me where to save files”

Now when you open attachments they will go to the /tmp folder.