If you can't retrieve any URLs at all, you've possibly been blocked (after a few email warnings - you do read your email, right?). Check your Nigel balances
here. If your any balance is way negative, pay the debt and you're away.
If Mozilla fails to run you probably have corrupt preferences, either delete or move your Mozilla settings. The settings are stored in your home directory "~/.mozilla". If you value your bookmarks or if you use the pop3 mail protocol just move this directory don't delete it. When Mozilla is first run it checks for the existance of the "~/.mozilla" directory, and if it doesn't exist it configures all of Mozillas proxy and mail settings for you.