Every Mozilla product has a build ID associated with it.
I think the index should become like:
|2.1 Firefox 3.1|
For Thunderbird the build ID is probably the value seen under Help -> About and the value you get in the header User-agent in every mail that is sent:
User-Agent: Thunderbird 3.0a1 (Windows/2008050715)
This string is only 8 characters long for official builds, though and probably 10 for older nightlies and 14 for newer?