I recommend the Firefox Aurora channel. It's a nice balance between bleeding edge and beta. That said, I've used Firefox Nightly for years and I've found bugs, as expected, but nothing critical.

One caveat is that some people are annoyed by the daily update notifications. If you set the about:config pref "app.update.silent" to true, Firefox should never show you an update dialog:

http://kb.mozillazine.org/App.update.silent