It is my understanding, from what I've read, that it will be very limited in scope. The OS will be designed for netbooks using a very limited version Linux with only enough to run the Chrome browser. Applications will be designed to be run off the browser similar to Google Apps. With that type of limitation, I really am not sure it will be capable of running Firefox, even with tweaking unless you recompile Linux with with whatever ends up missing.
It is yet over a year away, so as to be true vaporware with lots of speculation as to exactly what it will contain and thus I qualify my statement to be probably meaningless drivell. Regardless, Drivell or not, it is still my take on the product.