Doesn't matter much, as the number of people using Firefox has been steadily dropping for the past couple of years. It's not like this change will affect that; however, it does seem that Yahoo is a little late with this strategy, if they're trying to use to boost numbers.
There's always a slight chance that if a user that's used to using Google suddenly sees Yahoo is the default search engine, they'll get annoyed and switch browsers. But I suspect most people in that situation would either figure out how to change back to Google, or just shrug and say "oh well", as you said.
Either way, at this point, I don't see this helping Yahoo at all, and it could potentially slightly accelerate Firefox's fall. A lose-lose.