An interview with an anonymous Mozilla developer put forward the idea that the names “Firebird” and “Thunderbird” were for internal use only. This has since been verified by Mozilla developer Christopher Blizzard. The Mozilla Application Suite will continue to be known as “Seamonkey” internally. The new Web browser and e-mail client will be known as “Mozilla Firebird” and “Mozilla Thunderbird” respectively until the 1.4 release, after which they will become “Mozilla Browser” and “Mozilla Mail”.
The interview is worth reading. It seems to show that there is even a little confusion in the Mozilla development community. It looks like things are on track to become less confusing in the long run, but I don’t quite buy the excuse of “it’s just an intermal project name.”