Many talented developers are getting paid by Google to solely hack on Chrome - to add new features, to make it blazing fast, and to make sure it is well supported. The fact that it is open source is kind of a nice little bonus - Google-sponsored development is going to dwarf the community development, as long as Google is paying. Not only that, but any major community development runs the risk of being forked off into a less visible product if it doesn't conform to Google's direction. Google, after all, is not going to spend ad money on convincing people to download an offshoot of Chrome.

Mozilla does not pay a vast number of developers to head in one direction on Firefox - it asks the community nicely if it will head in one direction (because of Chrome's performance advantages, 4.0 will heavily emphasize that direction). By nature, this means that the Firefox codebase is going to be more fragmented than Chrome's. Things like comprehensive performance enhancements will be correspondingly tougher to do. The advantage of this is that there is a much less rigid steering committee - community developers on Firefox are free to go do whatever they want. The results are a slower, but more customizable browser.

I suspect that Chrome will probably end up with a bigger market share within 5 years, just because of Google's financial clout, but I doubt that Firefox will disappear completely.