> Any ideas as to where it could come from?
Google only needs Firefox as long as it's market share is significant. If Chrome becomes dominant and Firefox drops to something small (say sub-10%) then why pay them $$$. Based on the uptake of Chrome that may well not be too far off (although most Chrome growth so far has come at the expense of IE, Firefox has dropped a little)
http://en.wikipedia.org/wiki/Firefox#Market_adoption
> Any ideas as to where it could come from?
Make a better browser and drive up market share?
Many techies I know (myself included) are now advocating Chrome (or at least Chromium-based) browsers to friends and family, traditionally I would have been advocating Firefox. Why? Primarily browser speed. Two years ago FF was simply getting slower and slower, and it had a questionable security model FWICT. Chrome came along and was more responsive for "real uses" (who cares about Javascript benchmarks, fast DOM manipulation and page rendering is what matters most), and the one process per thing approach seems exceptionally sensible. Granted Chrome is not without its bugs, but then FF seemed to be on a downward slide at the time too ...