Well if the message is coming from firefox it is unconnected with avast. a screenshot of the message window (cropped to see just relevant data) might help to pin down what is causing the message.
Firefox uses a safebrowsing function that alerts on sites in its database - interestingly this database is based on google safe browsing.
If you have the sec_error_unknown_issuer error in Firefox, please add mode details: what version of Firefox and what Windows do you use.
Thanks.
I'm using Windows 8.1 (Windows Blue) OS. Just like what I've said, both of my web browsers are up to date and disabled unnecessary plug-ins and extensions. Once again I activated the HTTPS scanning service to see the outcome. Here are my screen-shots:
In Mozilla Firefox (33.0.3), I was trying to access https://www.facebook.com (https://www.facebook.com) and here's the result:
(http://i37.photobucket.com/albums/e91/chumgirl_18/firefox.png) (http://i37.photobucket.com/albums/e91/chumgirl_18/Screenshot2014-11-10210417.png~original)
(Please click the thumbnail above to see the full image)
On the other hand, this is what I get in Google Chrome (38.0.2125.111):
I was trying to access https://twitter.com (https://twitter.com) homepage but as you can see it couldn't load properly. It turned out like a text-based web browser and unable to load javascripts and images.
(http://i37.photobucket.com/albums/e91/chumgirl_18/chrome.png) (http://i37.photobucket.com/albums/e91/chumgirl_18/11-10-20148-40-22PM.png~original)
(Please click the thumbnail above to see the full image)