In toolkit, there is the string settingsChange.label[Unix|Mac], which appears when one sets a permanent option in the download file dialog. If one clicks the checkbox ‘always use this option with files of this type’ (or something similar), there appears the above string. This string is correct in FF, but not in TB, where it should refer to the Attachments section of the preferences.
Things have changed when an attachment is downloaded. Now it appears a directory location dialog and it just gets downloaded. The string seems not to exist also, on the toolkit nor in the locale directory in /mail. Bug 395317 removed them. So, I mark it as Resolved Fixed as when this bug was filed, it was ok.