I love Firefox's Rikaichan extension, it shows the meaning of Japanese kanjis when hovering the mouse over them.
I am writing a lot of text in Japanese, so to benefit from Rikaichan I edit in a HTML-textarea page I created locally. I open this page in a Firefox tab when I want to write a text. The problem is that the risks of loosing the text is high.
A better alternative would be a XUL-based (Firefox extension?) Notepad-like simple text editor.
Thunderbird is too heavy. Gmail's Compose window is not convenient and inserts undesirable carriage returns.