"...We'll probably do an Alpha when we're far enough on all the tasks that we can encourage testing by a broad audience, and we'll go into beta when have the feature set completed and only stabilizing and cleanup work is left. Until then, those of you who dare to possibly test unstable, not completely working code and have good backups of their private data so that possible damage to it is no problem, can test our trunk nightly builds..."

http://ftp.mozilla.org/pub/mozilla.org/seamonkey/nightly/latest-trunk/