Davide Pesenti
 Tue, 30 Sep 2014 12:56:50 +0200 
I run into the common problem described here: https://support.mozilla.org/en-US/kb/connection-untrusted-error-message#w_the-certificate-is-not-trusted-because-it-is-self-signed

(site name) uses an invalid security certificate. The certificate is not trusted because it is self-signed. (Error code: sec_error_untrusted_issuer)

Since the solution found there did not work, I try to describe here how I did it:

Open firefox/iceweasel's Options window, which is under "Edit" in the GNU/Linux versions, or under Tools in the Win version
Go to Advanced -> Encryption and click on "show certs".
Go to "Server" tab and click on "Add exception".

In the dialog window that opens, write the url of your site and click on "Get certificate" and then click on "Confirm security exception".

That should do the work.

You may try to find the right menu items, since I'm translating it on the fly from the Italian version.

Have fun!