When it comes down to it, each side really only has one thing that the other doesn't. However, those "things" are both very big features, so it comes down to a matter of personal preference.
Camino is better-integrated with OSX. A fair portion of this is to be expected, what with the native UI and all, but for example, the Preferences dialog actually looks like a Max OS X Preferences dialog (not that Firebird's is bad, but it looks like a Mac/Windows hybrid).
On the other side of the fence, Firebird supports XUL-based extension; Camino does not (and is unlikely to ever do so). This is, in itself, an extremely powerful feature, and allows for many things that Camino simply can't do at the moment, and some which Camino will probably never do (like the awesome ChatZilla IRC client).
In the end, it's your call. If you can do without the tight OSX integration, go with Firebird. If not, you won't lose all that much by going with Camino.