It looks like Mozilla is heading to court for a showdown with Yahoo Holdings and Oath over an alleged breach of contract.

Yahoo Holdings and Oath, the company that runs the Yahoo search engine and which is owned by Verizon, filed a complaint against Mozilla with the Superior Court of California on Dec 1, after Mozilla revealed it was changing its default search engine. Today, Mozilla announced it was fighting back by filing a counter complaint.

The story so far

Mozilla unveiled its new lightning-fast Firefox Quantum browser three weeks ago, and the reception has been one of widespread adulation. Firefox, it seemed, was back with a bang. But alongside the launch, the company announced that it was ditching Yahoo as its default search engine in favor for the infinitely more popular Google. Mozilla, you see, had inked a deal with Yahoo in 2014 to make it the default search engine in the U.S for a full five-year period.

Though users can switch their default search engine manually, having a search engine featured by default on a major browser like Firefox has a sizeable impact — five months after the Mozilla / Yahoo deal was inked, Yahoo said that its search volume reached a five-year high. And Google became pretty desperate to get people to switch their default search engine back — even placing messages at the top of search results.

Last year, as Yahoo was preparing to sell to Verizon, a notable clause in the contract between Mozilla and Yahoo emerged. It effectively committed the acquiring company to pay Mozilla $375 million per year through 2019 if Mozilla wasn’t pleased with its new partner. It also allowed Mozilla to walk away from the deal completely. Verizon isn’t renowned for its commitment to search, and it doesn’t seem like the most natural bed partner for Firefox, which may be why Mozilla pulled the plug on its Yahoo search deal.

Many of the specific details of the counter complaints are redacted in the court filings, but Mozilla’s Denelle Dixon, chief business and legal officer, said that all the company had done was exercise its contractual rights, “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.”

Yahoo’s acquisition by Verizon wasn’t in the best interests of Firefox users, according to Dixon, in terms of the search experience they would be presented with. And to rub salt in the wound, Mozzilla is pushing Oath — Verizon’s digital content subsidiary — to cough up the money it thinks it’s owed until 2019. That could work out to around $750 million, plus however much Mozilla is earning from Google as its new search partner.

Here is Mozilla’s official statement in full: