Well I have the Firefly, not the HD, and there's no update yet for my ereader model so this might be the reason I'm not able to find a fifth icon. I have "Retrieve Words" (the sheet of paper with the looking glass), Explanation (question mark), Similar Words (the three bubbles), Dictionary list (the book icon) and Adjust Position (the two curved arrows). No D icon whatsoever though. And no dictionary icon on the toolbar, not in FB Reader nor in CoolReader. Wait, checking the new PDF Reader... Yup, it's as you're saying, fifth D icon.
Thanks for the hint.