O.K. I went to the Mozillazine forums and got help with this. I found
it can be done using a VB script instead of a link. In the SendTo
folder create a new text document and paste the following into it:
Option Explicit
Dim objShell
Set objShell = CreateObject("WScript.Shell")
objShell.Exec("<path>\thunderbird.exe -compose subject='Requested
document',body='See attached',attachment='" & "file:///" &
WScript.Arguments(0) & "'")
Set objShell = Nothing
Change the subject and body text as desired, or leave them out if you
want them to be blank. Note that single quotes (apostrophes) enclose
the text arguments for subject and body.
Rename the file with a .vbs extension. Then in the send to... context
menu of Windows Explorer you should see the name of the file you just
created. Use that to open a compose window with file attached.
HTH,
Gabor