plugin=diff is not implemented.
http://yellow.ribbon.to/~tuotehhou/index.php?%2BThunderbird%2B%E3%83%87%E3%83%BC%E3%82%BF%E3%81%AE%E3%83%90%E3%83%83%E3%82%AF%E3%82%A2%E3%83%83%E3%83%97
plugin=diff is not implemented.