unresponsive script error citing mailWindow.js - with McAfee Internet Security Suite. Cause McAfee

Since installing McAfee Internet Security Suite provided by AT&T, I have been having problems with Thunderbird. It takes a long time to download messages and hangs up with Not Responding in the title bar. Then after it finishes downloading messages finally, the bog down continues. When trying to open or delete messages i get the Not Responding. And sometimes get an error message: Warning: Unresponsive script A script on the page may be busy, or it may have stopped responding. You can stop the script now, or you can continue to see if the script will complete. Script: chrome://messenger/content/mailWindow.js:180 once i stop the script the message will delete or close, but the problem continues. I tried starting Thunderbird in safe mode but it does the same thing. I have no add-ons anyway.