Looks like it only adds a "search merriam-webster" option in context menu for selected text. I wish description was more explicit about this

Great add-on ! It would be perfect if we could assign a keyboard shortcut (e.g. ALT+W) to trigger the Merriam-Webster tab opening with the word we (completely or partially) selected.

Muito útil e rápido para buscar o significado de termos em inglês.