Mozilla will upgrade the stable channel of its desktop browser to Firefox 20.0 today. The beta release notes are the source of our information as the final release notes have not been published yet.
One of the biggest improvements is the new per-window private browsing mode that all Firefox users can make use of now. This is basically the same feature that browsers such as Chrome or Opera have been offering for a while. Instead of having to switch to private browsing mode and closing all regular browser windows and tabs in the process, Firefox users can now launch a private browsing window next to the existing windows so that they can work in regular and private browsing windows at the same time.
rest here  (Windows version alert)