Wiktionary Popup от ivar.hammarberg

When you select a word with thjs addon enabled, a small symbol will appear next to the selection. Click this symbol and you will be able to access the definiton of the word as well as navigate through Wiktionary using regular links, all within the popup.

If you want to open the definition in the "real" Wiktionary, there is always a link available. The addon can easily be configured to blacklist or whitelist certain languages as well as hide or show things such as translations and derived terms.