I am trying to find a plugin or another method to block Firefox with a master password, so that a user can only use Firefox if the correct password is entered.
Can you suggest a plugin or if Windows 7 has this kind of blocking feature built in?
It's likely you're going about your "trying to lock down the machine" approach in a non typical way, and will struggle to correctly secure it.
You're probably better off having additional login accounts with restricted access, that way those users will not have access to what ever it is you're trying to protect in your specific firefox install (history, saved passwords, etc).
If your concern is only for:
Well, you can use the UAC feature of Win7 to block it. But then you have to provide password for each application u run.. Anyway, its the best hand you have..
Or, another scope is to use a firefox that'll require users other than you to enter a password when they try to access your saved password or bookmarks etc(i.e. your profile data).. To set that,
You'll get a prompt saying what you need to do next..