Playing DD on a Macbook in Firefox full screened, I've encountered two instances where the dialogue is so wide the button is hidden outside the Unity area. The first was in the initial tutorial, and reloading the page allowed me to move past it. The second is in the intro puzzle, and I cannot complete the puzzle. Several others have had the button half off the screen. In general, the dialogues are not centered and are too wide.
Here's a screenshot of the offending dialogue: http://i39.tinypic.com/8zqs9j.jpg.
(While writing this report I realized that <enter> will work, but the extra wide dialogues are still a problem)