Public Wifi connection

If the airport's Wi-Fi uses a captive portal, your Mac will first attempt to access the portal via a non-HTTPS website (captive.apple.com) to determine if that captive portal exists. It does this by trying to access the Apple servers. If it is successful, then no captive portal was found. If not, then the Apple Captive Network Assistant popup (basically a Safari mini-browser window) is trigged and the actual captive portal's sign-in page is displayed within that popup. At least, that is how it is supposed to work.

Ref: Captive Network Portal Behavior - Wireless Broadband Alliance

To resolve this issue, try the following:

• Turn off Wi-Fi, then delete the following:
  • /Library/Preferences/SystemConfiguration/com.apple.airport.preferences.plist
  • /Library/Preferences/SystemConfiguration/NetworkInterfaces.plist
  • /Library/Preferences/SystemConfiguration/preferences.plist
• Turn Wi-Fi back on.

Did you open Safari, or your default web browser. Did a page load up?

Additional note: You posted in the AirPort category, Airport refers to a discontinued line of Wireless Access Points from Apple, not an Airport, as in where you get on a plane.