Most important feature that they added is mixed content blocker.
What is mixed content blocker?
If you are on pages with HTTPS at start of the URL and any content (scripts etc) linked from URL having HTTP in start instead of HTTPS, Firefox will block it because it is not encrypted.
I forgot to tell that they also updated their logo. New options panel is created for developers that work in Developer Toolbox. They also removed support for <blink> element and also removed blink effect from text decoration. Network Monitor is added in developer toolbox.If you are on pages with HTTPS at start of the URL and any content (scripts etc) linked from URL having HTTP in start instead of HTTPS, Firefox will block it because it is not encrypted.
Default search provider is Google in Firefox but if you want you can change to any other search provider across the entire browser. Also included lots of security fixes and also removed lots of bugs in new Firefox.
New in Firefox for mobile:
Now when you'll scroll down Firefox will hide navigation bar. Subscribe to feeds by taping on address bar for long time. Firefox will display URLs of Web Pages instead of page titles if you want. Specify which search engine you want to use in your browser. Switch-to-tab functionality is also added.There are lots of more things to get benefit from. Just upgrade to Firefox 23, if you don't have Firefox installed then just go and get it.