Otel Consulting has developed a fix for firefox 7 for drag and drop zimlet for Zimbra 6.x systems. Have not tested it under Zimbra 7.

Corrects the following issues:
  • java.io.FileNotFoundException: zimlet description not found: com_zimbra_dnd.xml
  • undefined file description after upload
  • "Critical - Error uploading file" dialog appears and the "Attaching files" line sort of sits there and spins

More info and instructions can be found here.
Zimbra 6 Drag and Drop – Firefox 7 Compatible Fix | Otel Consulting Blog