I had the same issue with Thunderbird. However the ~/Library/Preferences/com.apple.internetconfig.plist file had no "Download" key in it, but there was another file named com.apple.internetconfigpriv.list that had this key. I could not delete just the value of this key, because Property List Editor complained that an empty value could not be accepted (it said it was not valid). So I deleted the plist file alltogether and it really did fix the problem. I had to choose my download folder manually on the next download, but after that the program remembered the folder location as expected.

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followed fyngyrz directions. found in the plist:
"x-stuffit^DownloadFolderoHelperExtra "

after removing the ^
and resaving the plist
it is working.
thanks

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