As mentioned in the Three Pillars of Firefox from July 2015. The Uniquely Firefox Pillar hinted towards an improved private browsing mode. In the past Private Browsing would not show up in your history, or keep cookies and temporary files. So, now Mozilla has expanded on what happens in a private browsing session and has rolled this out in the current Firefox 40 Beta.
Our hypothesis is that when you open a Private Browsing window in Firefox you’re sending a signal that you want more control over your privacy than current private browsing experiences actually provide. The experimental Private Browsing enhancements ready for testing today actively block website elements that could be used to record user behavior across sites. This includes elements like content, analytics, social and other services that might be collecting data without your knowledge.
Once again, this leads into marketing about the Extension Signing how it is this great thing and is suppose to make users safer by not allowing unsigned extensions to be installed. After all Mozilla’s philosophy is the user can’t protect themselves and an unsigned extension is an unsafe extension. The reality is an unsigned extension is all likely a safe extension that developer can’t get through or gave up on all the approval hoops.