I'm one of those people that likes to keep up. Nightlies often break, but they also have the absolute latest features. It also gives you the opportunity to comment on new features as they are added, and perhaps shape them to work in a way that others will appreciate.
So, if you want to try them out, realizing that they could '
kill your dogs', you can get the nightlies here:
Index of /pub/mozilla.org/firefox/nightly/latest-0.9
Also, you should keep an eye on these pages, so that you know what was changed and if others are having problems with them:
The Burning Edge - Mozilla Firefox nightly build blog
Firefox Builds - MozillaZine Forums
And yes, you spelled it correctly. It's called a nightly because they build them every day, usually early afternoon EST.