While it does give quick access to several different dictionary sites, this Firefox extension doesn't integrate as well with the browser as other Firefox add-ons do.
Mozilla Internet Dictionary works outside of the browser, opening a separate dedicated window directly above the Firefox window. The interface is understandable but it lacks polish and appears a bit amateurish. It includes eight tabs and a small field to enter words. You have to enlarge the window to see the definitions from sites that include Merriam-Webster, Encarta, WordReference, and Ultralingua. The only integration option it offers is the capability to add a toolbar button to Firefox, but that still opens the big window above the browser. That seems odd considering that there are plenty of toolbar options that offer access directly from the browser window, while still allowing you to look up words on several sites of your choice.
Mozilla Internet Directory works as promised, but its approach seems clunky. Still, if dictionaries are really your thing, it doesn't require any special skills to use and it's offered at no charge so there's no risk in giving it a try.
Mozilla Internet Dictionary is a tabbed dictionary browser using the language dictionary resources of the Internet. You can start Mid from your Firefox's Tools menu. You can also add a toolbar button for easier access: After installing and restarting choose customize toolbar and drop Mid's icon upon the toolbar.