On my machine, Thunderbird has the latest version, but Avast Software updater incorrectly flags is as out of date, asking for the same version to be installed.
Win 7 Pro
Avast 2014.9.0.2018
Virus Def 140616-0
if you check … ad remove programs > do you see more then one listed?
also see FAQ section here http://www.avast.com/en-gb/faq.php?q=software+updater#searchForm
avast! 2014: Resetting the internal database of the Software Updater http://www.avast.com/en-gb/faq.php?article=AVKB110#artTitle
did this help?