If you don't know the password for your computer, you are going to need to wipe it and reinstall. You look to be using ubuntu, so go to http://www.ubuntu.com/desktop and download a fresh, updated copy, use it to wipe your hard-drive and re-install. Back up any personal information first, as you will lose everything, including your Firefox profile. After you do that, you'll create your own password for the computer, and you can update Firefox and all other software on the machine. It's a VERY bad idea to use any used computer without first wiping it, you don't know what sort of malicious software the previous owner could have left on there.