Thunderbird on Macで添付ファイルがデスクトップに保存される
go to Preferences->General (in Thunderbird go to Preferences->Attachments) and click the radio-button next to "Save files to", then click the "Choose..." button and select the folder where you want to store "temporary" files when they are opened. After that, select the radio-button next to "Always ask me where to save files".
