From the Release Notes:
“Thunderbird is a full-featured e-mail and newsgroup client that makes emailing safer, faster and easier than ever before more. More information about Thunderbird is available.
“Given the maturity of the underlying Mozilla code, Thunderbird is very usable. However, it is considered a Technology Preview, and as such is assumed to have defects. See the License Agreement for more information…”
News source: Mozilla Tunderbird Project Page