Can we get a little more info? Like the version of Firefox and what operating system you are running. Have you tried running it from the command line in safe mode? Do you see anything in your virus software logs that might be preventing it from running? If you are running widows, try going to safe mode in windows and then running firefox.

Some problems occur 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. For detailed instructions, see

Configure firewalls so that Firefox can access the Internet. {web link}