Logged in as a standard user and tried to insert the lead record using data import wizard. I flowed all steps which were mentioned step by step but at the end, I am getting "congratulations your import has started" popup window I clicked "OK" but it will closing please refer the attached screenshot.
As what Satheesh has said, the problem that you're seeing might a browser issue. A more important question to ask is: Did your lead records go into your org successfully? That's what matters at the end of the day.