I'd say it's pretty obvious. Think of the wealth of information available through a browser? Most visited websites, favorites, the kind of information you could amass on a user would be huge. This is of course all tied up with improving their adverts. I'm guessing that mozilla would balk at the idea of handing over user data.
Several bloggers have said that the browser features an option whether to send data to Google or not, kind of like the Microsoft Customer Experience thing that collects anonymous data. However none of them had sources, so they can all just be rumors. It will all be clear in a few hours.