After months of discussion and further months of legal investigation, we're (phoenix team) finally comfortable moving forward with new names. The new name for the Phoenix browser is "Firebird". The documentation and product strings will be updated soon. In addition to securing the Firebird name, we've also got the OK, from those contributing legal resources, to use the name "Thunderbird" for an email client. Hopefully this will be the end of naming legal issues for a while.
News source: Mozillazine Forums
Note: Firebird Image © 2003 L. Cook