Lightspeed is currently not a product. It's a collection of sketches and thoughts. It is also NOT the next version of Firefox. While some ideas might find their way into mainline Firefox at some point, many of the assumptions on which Lightspeed is based are the exact opposites of Firefox core values (e.g. no settings or customization in Lightspeed).
More than anything else, Lightspeed helps us think outside the box that Firefox is. It's a place where we can dare to explore more radical thoughts like not having any settings or or even menus. Having constraints like these stimulates creativity much like, for example, the character limit on tweets forces you to make your message more concise.
Ideas are worthless when they just exist in your head. Sketching out Lightspeed has helped us to make make lots of ideas more tangible, so they can be evaluated.
That being said, just reading through this thread has sparked some interesting new thoughts – we'll keep experimenting :)