THIS EXTENSION IS OBSOLETE.
The change described below has been backed out. Firefox 4.0 / SeaMonkey 2.1 work again the same way as Firefox 3.6 / SeaMonkey 2.0 do. That is, they all set the current time as downloaded file's modification time. See bug 178506 for details.
See Preserve Download Modification Timestamp extension if you want to preserve the timestamp sent by server.
Firefox 3.6 / Seamonkey 2.0 and earlier used set downloaded file's modification time to current time. In Firefox 4.0 / Seamonkey 2.1 the behavior has been changed. If server returns a timestamp telling when the file was last modified (Last-Modified header), it is used instead. There is no option to get the old behavior back.
Download Timestamp to the rescue. This extension brings back the old behavior and always sets modification time to the current time.