Rated 5 out of 5 stars

Great work. Dictionary works well.

Solution for installing on latest Firefox 4 nightlies:
Download the .xpi file (right click "Add to Firefox" button, select Save link as...). Open with 7-zip or your favorite ZIP extraction tool and extract the .aff and .dic files into the Firefox dictionaries folder. (located in the firefox installation folder).

This review is for a previous version of the add-on (