Yep, I've been using the nightly builds from Mozilla for a long time now, probably nearly a year. Right now this is being typed on the current nightly build of Firefox 15, 15.01a. These are the nightly builds of the future versions of Firefox, and so far they have been as stable as the production version of Firefox. I love 'em.
These builds will update themselves nearly every night, and sometimes the features will change from one to the other. It is usually minor. The features are usually way ahead of the production release of Firefox and really neat if you dig into what it can do. There are usually tweaks to the rendering engine too, and these make it much faster than the already fast production release.
Have fun, I love these, and I hope you will to. If you don't, you can always install the current version of Firefox over it and go back to that.
Edit: If you use Nightly, please enable the reporting features, they don't pass anything of yours at all to Mozilla, just report on browser performance and stability so Moz can find and squish any bugs. That is the whole purpose of letting the public use these: faster and much more thorough testing. But don't worry, they work beautifully, and I've twice emailed Moz asking why they don't just release the current Nightly build because it is so stable and fast.