Google Define is a search plugin for firefox. It is a very small modification of the default google firefox search plugin. The arguments will be passed to google with the “define:” prefix .
It is composed of the xml file (use “save as”, do not try to see it in a browser: it start with a #) and the icon (). you have to place them in the “searchplugin” directory of the firefox installation.

It works well if you install google toolbar : you have two search box : one for google, one for google define ;)