As Mark Banner points out, Thunderbird is string-frozen for its beta 1 release as of 23:59 PDT yesterday. This means that any l10n strings in the mail/ and editor/ui directories in the comm-central repository are now frozen.
The code freeze for beta 1 will be on Tuesday 23rd September at 23:59 PDT.
There are a number of new features we landed just before the string freeze, some of these still need some polish, more details about them later.
We will be having a test day for beta 1 on Thursday 18th September. It will be covering the new features and checking for any significant regressions before we hit the code freeze date, more details coming soon.