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?
Which is a great choice IMO. Open source software needs funding, and I'm always a fan of projects finding ways to be self supporting rather than relying solely on donations.
Changing default search is trivial enough for anyone who cares to, and if they did go with Duck Duck Go or similar as the default, I think you'd likely find many/most users changing the default search to Google anyway.
"We exercised our contractual right to terminate our agreement with Yahoo! based on a number of factors including doing what’s best for our brand, our effort to provide quality web search, and the broader content experience for our users. We believe there are opportunities to work with Oath and Verizon outside of search," Mozilla Chief Business and Legal Officer Denelle Dixon said in a statement.
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?