Webform: prepopulate fields through URL, "page not found"
Here is the answer.

It seems clean URLs are not enabled in your site. So here I'm giving the solution in both ways

Assuming that your Field Key  for First Name field is first_name

If clean URLS not enabled. 


  http://localhost/test_site/?q=content/prepopulate&amp;first_name=miranda


If clean URLS enabled


  http://localhost/test_site/content/prepopulate?first_name=miranda


I've tested this on my environment and works perfect. Hope this helps