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 toobar and drop Mid’s icon upon the toolbar. The following translations are included: de-DE, en-US, es-ES, es-AR, fr-FR, hr-HR, hu-HU, it-IT, ja-JP, nl-NL, pl-PL, pt-BR, zh-CN.