Yahoo Holdings and Oath filed a complaint against Mozilla and in response, Mozilla filed a cross-complaint against Yahoo Holdings and Oath for breach of contract.

On December 1, Yahoo Holdings and Oath filed a legal complaint against Mozilla in Santa Clara County court claiming that Mozilla improperly terminated their agreement.

While this is a legal matter and much of it is confidential, Yahoo claims that Mozilla terminated a long-term strategic agreement following the sale of Yahoo's operating business to Verizon Communications Inc. The agreement was related to the inclusion of Yahoo search as a default search engine in the Mozilla browser.

In November 14 2017, Mozilla signed an agreement with Google, to make the default search provider in the countries where Yahoo was the default search provider (U.S., Hong Kong and taiwan), and Mozilla launched its new version of its browser.

"We recently 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," Mozilla said.

Immediately following Yahoo's acquisition, Mozilla says it undertook a lengthy, multi-month process to seek assurances from Yahoo and its acquirers with respect to those factors.

"When it became clear that continuing to use Yahoo as our default search provider would have a negative impact on all of the above, we exercised our contractual right to terminate the agreement and entered into an agreement with another provider," Mozilla added.