Everyone likes private browsing, right? Well one gripe that has been with Firefox since the feature was added was you can't have both a normal and private window opened at the same time. What would happen is Firefox can save your open tabs, and then close them and enter private browsing mode. When you were finished, those tabs could be reopened and reloaded again. With this update, which is currently in the nightly builds, you can finally have both types of windows open at the same time, something that competing browsers have had for quite some time.
While this seems like a feature that does not require a lot of effort, the Mozilla developers have been working on this for a long time now, and are putting serious development effort into this. For now, they have a experimental builds out that include this feature. You can get these builds by clicking here.