Is Firefox using data connection or wireless connection when it's browsing the Internet?
Firefox is using the active connection that is activated in the moment of browsing. It does not have specific options to select Data or Wifi. It will use Wifi if you're connected to a wireless network or Data connection if you're using the Internet from your device.
Any internet browser requires internet connection while surfing the net. But while you are seeing offline contents such as a saved page Firefox does not require an active connection.