Based on lots of user feedback, I have improved the define word plugin for FireFox slightly – please use the comment section to suggest other improvements. The new version 0.3.1 can be downloaded here.
This version adds an option dialog where you can choose whether to use google.com, yahoo.com, wordsmyth.com, dictionary.com, wikipedia.org or a custom search string to look up the definition. It also removes one of the two pop-up menu item links as it is now an option whether to open the definition in a window or a new tab.
It has been tested with FireFox v1.0 on Windows and Linux – and probably does not work in Mozilla on any platform. I may look into this later.
To install it, download the .xpi file to somewhere, and then drag it onto the FireFox window.