First thing... You can't rely on software to protect you from the web. Disregarding ethical concerns, if a movie, software or whatever wants you to install something to use it, you always decline. There's no legitimate reason for that to happen, and it should be a huge red flag that you don't have what you think you have.
For Firefox, try doing a complete removal in Synaptic, then check in /home/.mozilla and make sure there's nothing related to Firefox in there. If there is, delete it. Then reinstall Firefox through Synaptic.