Firefox users may be happy to see Google as the browser's default search engine again. But the move has sparked a legal battle between the browser's developer, Mozilla, and Yahoo.

The two companies are now suing each other over a 2014 deal that made Yahoo Firefox's default search provider. That deal was reportedly quite favorable to Mozilla; it allowed the company to back out of the deal—and receive an annual payment of $375 million through 2019—if another company acquired Yahoo and Mozilla found the new partner to be unsuitable.

Last month, Mozilla decided to trigger the company's contractual rights and terminated the deal, following Verizon's acquisition of Yahoo. This came as Mozilla launched Firefox Quantum, a major update to the browser that has been earning rave reviews.

On Tuesday, Mozilla detailed the decision in a blog post, saying it spent months studying how the new Verizon-owned Yahoo might affect Firefox's web search, the user experience, and the brand. In the end, Mozilla concluded that sticking with Yahoo would have had a "negative impact."

In response, Yahoo filed a legal complaint against Mozilla in a California court, claiming breach of contract.

"Yahoo has suffered and will continue to suffer competitive injury to its business and reputation, among other harm," the complaint says. The lawsuit is demanding that Mozilla pay damages.

On Tuesday, Mozilla filed a counter-claim in court to force Yahoo to hold up the terms of the deal, and pay up.

"The payments owed by Yahoo are key to financing Mozilla's efforts to launch the new version of its flagship product, Firefox," the complaint says.

The document adds that Mozilla took a high risk by choosing Yahoo as its default search provider, so it sought "contractual protections." It also blames Yahoo for dragging down Firefox usage, by failing to improve the company's search engine.

Mozilla also voiced concerns over Verizon's ability to protect user privacy, citing the company's $7.4 million settlement with the FCC in 2014 for using customers' personal data for marketing purposes without their consent.

"Verizon's policy positions are also diametrically opposed to Mozilla's positions on core issues such as net neutrality and cybersecurity," Mozilla—a big support of net neutrality—said in the complaint.

Oath, a Verizon subsidiary which controls Yahoo, has not yet commented on Mozilla's counter-claim.

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