...was released about an hour ago.  Beta 1 was nice because it added inline spell check and things generally felt a little faster.  However it crashed frequently.  Hopefully beta 2 won't crash so often.  They updated the skin between beta 1 and beta 2 as well:

http://www.mozilla.org/projects/bonecho/releases/2.0b2.html

Running it now as I compose this post and it already caught a typo.

This feature is hot:

Client-side session and persistent storage:  New support for storing structured data on the client side, to enable better handling of online transactions and improved performance when dealing with large amounts of data, such as documents and mailboxes. This is based on the WHATWG specification for client-side session and persistent storage.