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 ...

..freshly upgraded to Firefox 7, but apparently the 'HTTP://' is gone in the addressbar. If you're looking under 'Options' to get it activated again, you're wrong. So quickly the steps to re-enable it again:

• open firefox
• browse to 'about:config'
• search for 'browser.urlbar.trimURLs'
• change the value ...