Addendum to previous: Rated 4 out of 5 stars
Addendum to previous:
A tooltip for multiple kanji is not needed.
Used with Rikaichan this addon ROCKS!!!
If there were an option for multi-kanji rather than just single kanji word equivalents I think it would be perfect. I would spend more time with Kanjilish than my text books.
BTW, This addon is useful not only for the study of Japanese but also for English.
Now, if somebody came up with a Rikai-chan equivalent for English to Japanese Firefox would be a language school.