I thought it was a bug in my install. You can't even close the top-most tour window, because of the covered prompt.

Apparently, the wide button where all the text is hidden sets Firefox as default browser. I accidentally clicked it when trying to give the prompt focus. Thankfully, OS X asked me to confirm that I wanted to do this.

I'm not sure what's worse, purposefully tricking me into setting default browser by hiding the text of the prompt, or starting with such a buggy first-run experience!