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 at github, and there was also a change recently (commit 7ee2458033819b1a3f0e74c1afadbc3e3922a898) which fixes this incompatibility. Strange indeed, that there’s not a newer version on addons.mozilla.org yet.
Anyway, I cloned the git-repository, compiled the code, installed in firefox, and voilà, everything back to normal, the plugin works again!
Here are the steps needed to compile it from GIT:
git clone git://github.com/foudfou/skipCertError.git
ls -lah ../email@example.com
or you can quickly download my precompiled ‘binary’ here (firstname.lastname@example.org).
Thanks to foudfou who coded this valuable plugin!