Did you start using Firefox because IE stopped working, or you were using FF and then IE stopped working.
If the latter, then uninstall FF and check to see if IE will work again.

Otherwise, check the settings for IE (Tools>Internet Options) to see that there's nothing blocking it.

Check your Advanced settings in your Firewall (both windows and McAfee) to make sure it's not blocking IE. Make sure it's not just blocking the ports that IE may use etc, but that the actual application itself is not blocked.

If you meant that IE stopped working in terms that it won't load ... then there may be a problem with the actual application itself. You may need to reinstall the it or get updates for it etc.
If you don't have the latest IE, download it to see if it makes a difference.

Cheers.

*updated*
try uninstalling FF, and then see if it makes a difference to IE.