Because at the time, the three "official" projects had inter-related codenames. The browser component came first and the name change from Phoenix to Firebird h

Because at the time, the three "official" projects had inter-related codenames. The browser component came first and the name change from Phoenix to Firebird had already happened, then email and calendaring were started which became Thunderbird and Sunbird respectively. The name generation scheme that they settled on was element + animal, and is the same reason they ultimately chose the name Firefox (element + animal).