It is a new release, fixed some security issues as below.
Fixed in Thunderbird 184.108.40.206
MFSA 2008-67 Escaped null characters ignored by CSS parser
MFSA 2008-66 Errors parsing URLs with leading whitespace and control characters
MFSA 2008-65 Cross-domain data theft via script redirect error message
MFSA 2008-64 XMLHttpRequest 302 response disclosure
MFSA 2008-61 Information stealing via loadBindingDocument
MFSA 2008-60 Crashes with evidence of memory corruption (rv:220.127.116.11/18.104.22.168)
If Thunderbird doesn’t update automatically, you can click Help > Check for Updates to update it.