There will likely be some people errantly suggesting this is due to search quality. It is not. Yahoo previously outbid Google approximately three years ago for that default search offering, and it's likely their arrangement has come to an end. One of Mozilla's strongest sources of funding comes from selling the default search engine spot to the highest bidder.

EDIT: As an interesting note: Yahoo's deal was for five years, not three. I am guessing Verizon decided it didn't want to pay for Mozilla anymore, and cut it off early?