Are you implying that none of the code in Firefox or Chrome is going to be orthogonal to rendering web pages? Is this how you architect your programs?
However if you meant the spread of ideas can flow from Chrome to Firefox (or even Safari; WebKit is open source as well), then yes, you're right about that. But its highly likely that the underlying code for said feature will be the exact same in all three browsers.
Instead, the ideas will be taken and copied (and hopefully made better). But idea's aren't code. So unless you're talking about human memory (which seems highly unlikely given you said "…where Firefox can improve") I'm not quite sure what benefits in terms of memory usage Firefox (or Safari or even Opera) gets from this.
Also, if someone implemented my original proposal in this thread, this would be entirely orthogonal to any code currently in Firefox.
I'd be interested to see Firefox embedded in a "wrapper" VM, but I think you'll find it harder to implement than you expect. Please post to HN if you give it a shot.
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.