Rated 5 out of 5 stars

ITS REPLACEMENT FOR NEWER VERSIONS OF THUNDERBIRD IS FINALLY UP HERE :
https://addons.thunderbird.net/thunderbird/addon/10ten-ja-reader/

Will it be Rikaichan or Rikaichamp or 10-ten, it's basically the best addon for Japanese language ever and actually the best Japanese-English tool ever, even my Casio electronic dictionary doesn't give such complete translations most of the time, especially for specialized vocabulary, it can translate even phrases containing many words as one item, such as administrative stuff :
配偶者控除 => "tax exemption for one's spouse"
基礎控除 => "basic or standard deduction"
源泉徴収票 => "tax certificate slip / statement of earnings / tax-income certificate"

or many other medical, scientific, industrial vocabulary, or vocabulary used in formal mail correspondence etc.