ThunderBirthDay is marked as compatible with Thunderbird 3.1 since the release of the latter. This information is made available through the Mozilla Update System. Every time Thunderbird starts, it retrieves compatibility information from this system. If ThunderBirthDay should be disabled on your machine, saying that it is not compatible with your version of Thunderbird, just restart Thunderbird while you're connected to the internet. After the restart, ThunderBirthDay should be enabled again and working fine. If not so, please drop a comment below. You can read more about Mozillas Compatibility Updates here.