This week's book giveaway is in the OCAJP 8 forum. We're giving away four copies of OCA Java SE 8 Programmer I Study Guide and have Edward Finegan & Robert Liguori on-line! See this thread for details.
No, you can't control this - it's a setting under the control of the user. What you can do is detect if a non-Firefox browser is used, and in case it is -instead of the link- show a message to the effect of "you need to be using Firefox to proceed".