Firefox 20 - skip the SSL/TLS certificate error

As of Firefox version 20, the addon Skip-Cert-Error (which is a rewrite of 'MitM Me') doesn't work anymore. In fact there were some changes made in ffox in regards to the private-browsing-mode, which made the current 0.3.3 version incompatible.
Luckily the developer also has its own GIT-repository ...