Thunderbird is as Firefox also now available in a new version with extended support release. With version 17, there are no significant changes, although the mail program has a higher version number. This is also true for non-ESR variation that increases with the next update to version 18.
That Mozilla Thunderbird does not want to fully support, has been known since July 2012 . Therefore primarily error of Thunderbird 16 eliminated, including emails repeating issue. In addition, Thunderbird 17 work faster. Some security issues have been fixed .
Further information is available in the release notes of Thunderbird 17