Thinking about it, it makes no sense for Google to release a Web browser just for the sake of releasing one. Why hurt FireFox? Why risking releasing a product that will be scrutinized for security flaws? It just doesn't make any sense.
So why do it? The only sensible reason I came with is this one: they want a better "runtime" for their Gmail, Documents and other Web apps. Why? Creating complex Ajax apps cost too much right now and on top of that, the browser was not meant to host such complex apps. Google surely wants to reduce their development cost and they surely want to offer more functionality but they are limited right now by the platform itself, the Web browser. By releasing their own browser, they will control the platform.
So there it is, Google Chrome is not a browser, it's Google's application runtime. They are not going after IE or FireFox, they are going after Silverlight and Flash as an application runtime.
At some point, you'll see advanced capabilities with Gmail or Documents if you run them from Chrome and people won't complain because it's Google. Simply.
What do you think? Any other crazy thoughts?