If you can start Firefox in troubleshooting mode but not in normal mode, it's quite likely that your problems are caused by a misbehaving addon. Try starting in troubleshooting mode, going to the addons manager and disabling all addons. Then restart in normal mode.

If that makes the issue go away, you can try re-enabling your addons one after another, restarting after each one, until you have the issue again. The you know that that addon is misbehaving and can remove it permanently.

hope that helps, till

If you had an update of Firefox, the firewall may be the problem.

This problem occurs when your Internet security program was set to trust the previous version of Firefox, but no longer recognizes your updated version as trusted. Now how to fix the problem: To allow Firefox to connect to the Internet again:

• Make sure your Internet security software is up-to-date (i.e. you are running the latest version)
  
  
• Remove Firefox from your program's list of trusted or recognized programs, then add it back. For detailed instructions, see Configure firewalls so that Firefox can access the Internet.