Mozilla CFO Eric Muhlheim told a US court that if the Justice Department gets its way and bans Google from paying to be Firefox’s default search engine, the fallout could be existential.

“It's very frightening,” he said, warning that Mozilla could face a “downward spiral” so severe it would kill off Firefox.

The Department of Justice has already ruled that Google runs an illegal search monopoly, aided by sweetheart default placement deals on browsers and phones. Now it’s gunning to dismantle that grip. This includes a proposed ban on default deals with third-party browsers like Firefox and a forced breakup of Chrome.

But for Mozilla, that lifeline deal with Google is most of the balance sheet. Muhlheim told the court 85 per cent of Mozilla’s revenue comes from Google, with Firefox accounting for about 90 per cent of the group's overall income. Rip that out, and Mozilla would be forced to gut Firefox development.

That means fewer updates, less engineering, and a browser slipping into irrelevance. This, Muhlheim argued, would ironically strengthen the Big Tech chokehold the DOJ is trying to loosen.

Firefox runs on Gecko, the only browser engine not owned by a trillion-dollar tech leviathan. Muhlheim noted that if Firefox dies, the last real alternative to Chromium-based web domination dies with it.

And it’s not just browser wars at stake. Mozilla’s nonprofit arm, which supports open source tools and AI efforts like climate modelling, is funded by the profits from Firefox and those would vanish too.