Google/Firefox Co-Branding

If you open www.google.com with Internet Explorer, you get an ad for the Firefox toolbar with google built in.

If you open www.google.com in Firefox, you do not get the same ad.

Obviously, browser targeted ads are not new. What I find interesting about this instance is the co-branding emerging between FF and Google.

As Microsoft and Google continue to fiercely compete in the online space, how will consumers be affected? Will Google eventually support FF only? Will Microsoft only use MSN?

Taking this a step further, will some sites only be accessible by Google and others by MSN?

I know this is melodramatic but it is certainly a trend worth following.