Tired of the way Thunderbird and Firefox clutter the desktop of your Mac. I certainly was. Each and every file I opened was left on the desktop until I could no longer separate the important files from the thrash. So I tried to search for a solution.
I found it in a two year old bug report (some bug report). Seems like Mozilla only uses the Mac OS X standard user temporary directory; which is the desktop. So; how to fix it. Simply open Safari and open the configuration and change the download directory under the general tab. So easy; so far fetched :-)
If you don't like this fix you could choose another route by telling Firefox to delete downloaded temporary files on exit. I don't know if the same can be done for Thunderbird. I chose the first solution because I often find myself editing opened documents forgetting that I opened them in Firefox; and I don't want to loose the changes. Nevertheless; this comment shows how: some bug report comment.