I don't think it will change a thing, and let me explain why.
First, I use both Chrome and Firefox. I use Chrome on one desktop and Firefox on the other. I don't honestly like 1 more than the other. I seem to get faster browsing with chrome but firefox has got macros which I also use. Then theres the fact that FF got the spiggot bug a while back, I tried like 10 different anti-virus/spyware programs to remove it (all the top recommended) and it never went away. So I stopped using FF on that computer and replaced it with chrome.
Anyway, that has nothing to do with anything.
What we are really questioning here are long term habits. The SE a person uses is their long term habit. And some people do switch from time to time but most don't.
That's why I think this change will have minimal if no impact at all. If 10,000 people were using Google and Firefox tries switching them to yahoo, I think 9,980 will go back to Google. Even the so called tech illiterate. Cause all they have to do is ask their tech literate sons, daughters, sisters, brothers to switch it back for them and they will. I think that saying "old habits die hard" would fit perfectly here.
-BB