This is usually a issue cased by your internet service. If you are at home try switiching the router on and off and if you are at work you can ask IT to see if they can do anything. If that dosn't work if you havn't already tried I would try again once you switched off the antivirus. If that dosn't work try annother browser and see if the problem persists.

Hope this helps, Daniel

Hi gowanlake, you might also try turning off the Avast WebShield, or setting Avast not to filter your Firefox connections to see whether that makes any difference. There's info on that feature/setting in the following article:

How to troubleshoot security error codes on secure websites