Updating FF friequently, it was discovered, that it's default behaivour changed. Now, when SSL certificate broken for any reason, there is no more Add Exception button. I will not discuss if it good or bad, but this does not suit for me still dealing with a lot of self-signed certificates.
Go to about:config firefox low level configuration tool. Locate key browser.xul.error_pages.expert_bad_cert and double click on it to change value to true.