If you want, please post any likes/dislikes or feature requests for this add-on in the Reviews section.
It is not a dictionary to learn other languages and get definitions in English. It gives definitions in the language that the word comes from.
Double left clicking a word in a webpage can highlight it straight away, then just right click and select Define At Wiktionary to get the page inset. However, double clicking will add an extra space at the end of the word selected that can cause the panel not to display sometimes, so if you get that, then select just the word manually with the mouse. Click outside of pop up to remove it.
Go to the Tools, Add-ons, Extensions, area and click on the Options button next to this addon to access the settings. In here you can set the language you wish to use, anything from German to Japanese, as well as adjust the panel size and shortcut key. If you press CTRL-SHIFT + Shortcut key it will open your panel in a new tab.
You can also left click and drag any link inside of the panel to your Firefox address bar and drop it in to navigate directly there.
Hope you like this extension. Please review it if you have time.