Hint: In theory, you can use any (modern) browser you like. However, using Google Chrome or Opera is not recommended because they don't show "password hints" (custom text in the authentication dialog of the username/password levels).
Earn a certificate:
Solve the puzzle without cheating; positive levels, negative levels and greek levels. You will be told how to claim your certificate at the end.
WARNING: If you cheat, you might get trapped before finishing the game, with no chance to reach the goal.