The DOJ wants to bar Google from paying to be the default search engine in third-party browsers including Firefox, among a long list of other proposals including a forced sale of Google’s own Chrome browser and requiring it to syndicate search results to rivals. The court has already ruled that Google has an illegal monopoly in search, partly thanks to exclusionary deals that make it the default engine on browsers and phones, depriving rivals of places to distribute their search engines and scale up. But while Firefox — whose CFO is testifying as Google presents its defense — competes directly with Chrome, it warns that losing the lucrative default payments from Google could threaten its existence.

Firefox makes up about 90 percent of Mozilla’s revenue, according to Muhlheim, the finance chief for the organization’s for-profit arm — which in turn helps fund the nonprofit Mozilla Foundation. About 85 percent of that revenue comes from its deal with Google, he added.

Losing that revenue all at once would mean Mozilla would have to make “significant cuts across the company,” Muhlheim testified, and warned of a “downward spiral” that could happen if the company had to scale back product engineering investments in Firefox, making it less attractive to users. That kind of spiral, he said, could “put Firefox out of business.” That could also mean less money for nonprofit efforts like open source web tools and an assessment of how AI can help fight climate change.