I'm doubtful that google will actually make an effort for the code to be reusable in firefox specifically, while I'm sure that there's a possibility google can and will contribute lots back to webkit.

Sure, some of the codebase will likely be reusable in ff, but it's not something you should really count on. And even if it isn't reusable, you can probably take a hint on some of the more interesting features and reimplement them if it turns out the parts you care about aren't reusable.