Pushka,
If you are seeing this message in one particular forum, I suggest you contact the operators of that forum. If you are seeing it in FireFox but not IE, try posting a question in a Firefox-related forum (e.g.
http://forums.mozillazine.org/).
You should also check your Firefox add-ins (if any). If you suspect a particular add-in is causing the problem, try disabling it to see if the message goes away.