If your kids know a few things about computers, per say, any measure you may implement to prevent them from running Firefox but ''not'' other applications on your computer can be easily circumvented. This also applies to other browsers, such as Internet Explorer. Answer your question, I'm not aware that there is any way to lock Firefox with a password. I can, however, advise you to create a system account for your own and put a password on it. If you want, you can create another system account for your kids (however many you like, in fact) and you should also be able to prevent those accounts from accessing the Internet at all (not just through Firefox, but through any software). Since you're running Windows XP, you need to contact Microsoft for support. I can tell you, however, that if you set up your kids' account as a guest account with no admin privileges, it will be reasonably hard for them to circumvent your measures, even if they are experienced. You may also create a login password for your local network. If you are using a dial up or DLS modem, it's reasonably easy to do. If you are using a router, then it's probably a bit harder, but it's feasible, and will make sure that there is no way to access the internet unless you know the password (or unless you factory reset the modem/router). As before, you need to contact the relevant entities for support.