Hello there.
What you need to do is:
1) Let's say your WiFi connection is named "123ABC", when you click the wireless icon to find it, there's an option that says "Connect Automatically", check that option and then connect to your WiFi normally, windows will remember you password and your PC will connect to that network always (of course, if it is available)
2) Do what maguscreed suggested, add a Firefox shorcut to the startup folder and it'll open everytime you boot into windows