My guess is the other way around: Yahoo backed out of their contract with Mozilla. The biggest difference for either of these companies this year is that Verizon now owns Yahoo, and probably doesn't want to pay for search market share. If the deal's a yearly cost or something similar, the bill's coming up in December.
We don't know the exact details of the contract. And even if it was written as such, both parties could agree to a change in the contract that could allow either side to get out on certain conditions.
"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.