Mozilla

Maker of Firefox and of the Gecko platform, which also carries Thunderbird and the downstream builds — zen among them. Mozilla publishes the platform's engineering documentation openly, which is why this spoke can describe an engine's internals first-hand rather than through vendor marketing, as it must for most Chromium browsers.

Where the money comes from

The audited consolidated statements for 2024 (mozilla-2024-financials, T1) put numbers on what was previously unread here. Royalties — a share of the revenue partners earn from searches run in Firefox, not a flat payment for the default slot — were $498.2M, against $66.4M from subscriptions and advertising and a total of $680.4M including investment income. 86% of revenue from customers with contracts came from a single customer, roughly $486M, or about 71% of everything that came in. The statements never name that customer.

Against that, software development cost $290.4M. The engine is affordable inside the search payment and would not obviously be affordable outside it, which is the honest answer to "who pays for the independent engine": the vendor of the dominant one, indirectly, through a revenue share that shrinks as Firefox's share does.

The Foundation and the Corporation are separate legal parties that transact: the Corporation paid the Foundation $22.6M in trademark license fees in 2024, computed on its royalty revenue.

Still unread here: the search agreement's term and exclusivity, and what the US antitrust remedies against the counterparty do to it.