Just like Google Chrome, Mozilla have come out with a public development cycle for their future browsers.

What is Mozilla Aurora?
I like to describe it as the Alpha of Firefox. You can see new features that will be apart of the future release. There is a new update every 6 weeks. Just like any alpha or beta, it will be unstable.

But how about Mozilla Nightly?
It's the alpha of the release of Firefox in two release's time.
So, at the moment, we have a stable release of Firefox 4. Aurora is the alpha of Firefox 5. Nightly is the alpha of Firefox 6.

At first look, both look the same as Firefox 4.
So I ran Spectrum on both of the browsers


Both ran smoothly, however, once I opened a new tab in Nightly, it did lag quite a bit (Doesn't help that I am capped too)

You can see what is new in each of the future releases here.
https://wiki.mozilla.org/Features/Release_Tracking



To download Nightly
http://nightly.mozilla.org/

To download Aurora
http://www.mozilla.com/en-US/firefox/channel/

For more information
https://developer.mozilla.org/devnews/index.php/2011/04/07/new-development-channels-and-repositories-for-rapid-releases/
http://blog.mozilla.com/blog/2011/04/13/new-channels-for-firefox-rapid-releases/
http://www.webmonkey.com/2011/04/how-to-use-firefoxs-new-aurora-release-channel


PS: Just don't expect your Add-Ons to work

This thread was written and pictures uploaded in Mozilla Nightly.