There is no reason to replace mozilla some people preffer it some not. Just add Firefox and make it selectable in preffered applications.
Actually, there are several:
1. Mozilla (Seamonkey) is considered deprecated by mozilla.org. "Firefox is the future."
2. As a result of #1, more development happens for Firefox; bugs are more likely to get fixed in it.
3. No future Mozilla threads along the lines of: http://www.redhat.com/archives/fedora-devel-list/2004-July/msg00315.html
4. Shipping two mozilla.org branded browsers is double the work of shipping one. And let me assure you that shipping one is plenty of work.