Just think about it. This is the same market dynamic that has been eroding the revenues of TV broadcasters ever since Tivo provided an easy mechanism for skipping ads. Don't you think that if most web surfers were given easy options for turning off web ads and/or making themselves more anonymous online, they would take advantage of it? The two hurdles to users doing this currently are: 1) installing Firefox, 2) installing the right addons. Granted those hurdles are still too high for a large percentage of web surfers, but I believe that if the Google-Firefox alliance breaks down over Chrome, that second hurdle will eventually go away, because it makes sense from a user experience perspective to bundle those options with the browser.
And no, I have no evidence or special insight into what high-level strategic discussions are taking place at Google or Microsoft. I'm just going out on a limb and guessing that those discussions are not about Webkit.