The products are different. It's nice to have choices, just like it's nice that we don't all have to run Windows.
Mozilla is an "application suite" with a browser, e-mail client, HTML editor, and other goodies all in bundle. Some people have been using Netscape or Mozilla's e-mail client for years and don't want to change.
FireFox is "just the browser". Leaving out the other features makes it faster for those who don't need them.
Mozilla and FireFox are cross-platform: they work and look just about the same whether you run them on a Mac, Windows, or Linux. Camino is an effort to give FireFox a more MacOS X native look-and-feel. Because I'm embedded in a multiplatform environment, I must confess to never having tried Camino. But I think it's great that some folks care enough to develop it.
I guess that's the rub: the mozilla.org group isn't really a "company", with a fixed number of employees coding for money. It's a voluntary assemblage of people cooperating to build the products they want to use. If there were no Camino, there is no guarantee that the guys who are working on it would turn their efforts to developing FireFox or Mozilla. They are involved because they want an OS X native client.
Crikey