Do you have any use for the incoming data, other than to test for the CR? If not, just test for the CR like below:

DO WHILE !FEOF(dHex)
IF ComForm.Testcom.CommEvent = 2  &&This is triggered if the RThreshold is reached, which is 1 character
   StringBuffer = ComForm.Testcom.Input
   IF vbCrLf $ StringBuffer  &&If a Carriage return is received
      ComForm.Testcom.Output = ">[1]" + firstLine + vbCrLf &&sends a line from the Hexfile
      WAIT WINDOW firstLine NOWAIT
   ENDIF
ENDIF
ENDDO
fclose(dHex)
WAIT WINDOW "DONE SENDING ALL LINES FROM DOWNLOADERHEX.TXT"