If you just want to use this add-on, go to: https://addons.mozilla.org/en-US/firefox/addon/in-page-lookup/
In Page Lookup is an add-on that lets you look up a word on any page in the same page as you have highlighted.
To look up a word on Wikipedia – Just highlight the word and click ControlLeft+AltLeft+1 and an overlay of the Wikipedia article will appear as an in-page overlay.
To look up a word on Wiktionary – Just highlight the word and click ControlLeft+AltLeft+2 and an overlay of the Wiktionary article will appear as an in-page overlay.
To close the overlay, click ControlLeft+AltLeft+~ (the key to the left of the '1' key if you have a different keyboard that doesn't have the `/~ key)
This code uses the following open source code:
jquery-3.5.0
jquery-ui-1.12.1