Drag and Drop slide comes up as a pop-up/lightbox?

I encountered a strange issue. A student informed me that she did not have the submit button in the drag and drop question. After she sent me a screenshot, it turned out that the question slide came up as a pop-up/lightbox on her notebook.

This is a simple freeform drag and drop question slide (Storyline 360) and so far it worked for everyone the way it has to. I tested it in Chrome, Explorer, Firefox on Mac and Windows, Safari, and it works fine everywhere.

Students can get to that slide from a different slide in the scene by clicking on a shape that has the trigger "jump to x.x Drag and Drop" when user clicks on that object.

This student uses Firefox 59.0.2 on a Windows notebook (Lenovo Yoga 500).

Does anyone have any idea what the problem can be and how to fix it? Please help