After 1.5, the Mozilla suite is being broken up into pieces. The browser will be Firebird, the mail reader is Thunderbird. etc.
If you get the Mozilla Firebird 0.6.1 release I think it uses Mozilla/Gecko 1.4 for rendering. If you download the nightly versions though they use the latest Mozilla/Gecko.
My copy is built with 1.5b from Sept 4th.