DavidR, your post saved the day 
http://forum.avast.com/avt/avatar_18134.png
Showed my Avatar. But :
https://forum.avast.com/avt/avatar_18134.png
Showed : see screenshot
So Firefox says the connection is not secure, because the identity of the website can not be confirmed. Adding it as an exception solves the problem. The Avatars are back
But why do I have this problem only with Firefox, and not with other browsers ? Any help and/or info would be appreciated 
You're welcome.
It is something to do with your security settings importing https into an http page (mixed security levels, etc.). I don't recall when this use of http(s) for the location of the avatars happened or why, but it certainly had an immediate impact.
Firefox has different security settings, some say they are tighter others just say different. Very recently we had a case of avast alerting on an injected iframe on a web site, yet I and another user couldn't replicate it as we were using firefox and only IE8 (in this instance) seem vulnerable to this injected iframe vulnerability.
Well, funny thing is I don't browse the forum in https.
No you don't browse the forums in https, but the link to the avatars is https so you are effectively accessing a secure site/page and firefox expects a certificate to check. But for some reason it can't check it, as there is now a valid certificate for the avast forums.
If you connect to the forums from an https link in a topic reply notification email, then firefox would have validated the certificate before opening an https page. Since you are already on an http page, this initial certificate check wouldn't have taken place.
