title: Recovering Ubuntu after apt-get upgrade failed
body: <p>I was trying to install Firefox 5 using these commands -</p>
  
  <p>sudo add-apt-repository ppa:mozillateam/firefox-stable
  sudo apt-get update
  sudo apt-get upgrade</p>
  
  <p>The upgrade was happening, but I accidently killed the process, and Firefox 5 was up, so I didn't bother then. But the next time I restarted the system, I was stuck at the login screen. How can I recover it ?</p>