Why is Chrome and Firefox not supporting Win x64 bit yet?

Chrome used to crash frequently for me on multiple Windows 7 64-bit machines, but it runs rock solid now. It turned out the problem is that Chrome auto-updates flash, and their software frequently crashes on 64-bit windows. One one computer, I had to disable Flash altogether. On the other, it was enough to disable Flash hardware acceleration. In both cases, Chrome is now completely crash-free.

On your broader question -- why not native 64-bit app -- I think the key reasons are 1) level of effort, 2) performance and 3) plugins:

1) Supporting 64-bits means a lot more than recompiling. It will need to be a separate development effort, probably doubling the resources required for the project.

2) 64-bit pointers are twice as large, and 64-bit programs take up more memory. Due to caching, this also means that the programs run slower.

3) All the plugin partners face similar problems of effort and performance. A 64-bit browser will necessarily ship with a lot fewer plugins than a 32-bit version.

Right now, a browser maker looking at a 64-bit version sees a lot of effort for a slower version with less functionality. And that just doesn't seem like a good bet.

FF4 is rock solid too on x64, under Linux that is. Not sure about Chromium though, I have it but hardly use it.