If you experiencing Java Plugin has been disabled by default, you can fix it now. How to know your Java Plugin have a problem? Check it now:
Click "Firefox" tab > Add-ons > Plugins
Check your Java Plugin is run normally or not. It maybe because of the version. Whatever for security reason, Firefox will disable it by default. This waiting for new release or update then when confirmed, it will available again to the user.
Simply update here(Java download)