Mozilla and Yahoo have started a legal spat about the deal that existed between the two companies regarding the use of the Yahoo search engine in the Firefox.

Yahoo fired the first shot, filing a complaint that alleges Mozilla breached a contract that existed between the two companies by terminating the arrangement early.

Mozilla says that it was not only justified in terminating the contract early, but that Yahoo still owes it dosh.

Yahoo's complaint mutters about "breach of contract" and "breach of covenant of good faith and fair dealing". Mozilla hit back with language like "declaratory relief", "breach of contract re payments", and "breach of contract over search performance".

Mozilla terminated the agreement that it had in place with Yahoo in early November, with Yahoo having allegedly ceased payments after the end of October.

Mozilla says that it was unhappy with Yahoo search performance, and made several suggestions to its owners Oath and these were not well well-received:

“Yahoo Acquirer's response to Mozilla's concerns was in stark contrast to Yahoo's assurances from CEO Mayer at the time the Strategic Agreement was entered into. Yahoo Acquirer's leadership provided no vision, no structured, documented and vetted strategic path forward, and no assurances as to a commitment of the resources necessary to improve Yahoo Search such that it would meet.”

Mozilla said that while it is not possible to reveal full details of what is going on, it would like to try to be as transparent as possible. Denelle Dixon added: “On December 1, Yahoo Holdings and Oath filed a legal complaint against Mozilla in Santa Clara County court claiming that we improperly terminated our agreement. On December 5, Mozilla filed a cross-complaint seeking to ensure that our rights under our contract with Yahoo are enforced.

“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.

“ Immediately following Yahoo's acquisition, we 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.

“The terms of our contract are clear and our post-termination rights under our contract with Yahoo should continue to be enforced. We enter into all of our relationships with a shared goal to deliver a great user experience and further the web as an open platform. No relationship should end this way -- litigation doesn't further any goals for the ecosystem. Still, we are proud of how we conducted our business and product work throughout the relationship, how we handled the termination of the agreement, and we are confident in our legal positions.”