Google cannot afford to have Firefox go out of business. Because of the fact that every popular browser aside from Firefox uses Chromium engine, tons of cases will be opened against them for being a monopoly in internet browsing. Because of mainly this Google is the top donator for Mozilla by far.
This is also my take on the case. Sure Google officially pays that money to be the preselected search engine in Firefox. However, in my opinion, the amount isn't justified, and if this was only an economic matter, they'd have negotiated less by now. But they know that as soon their money dries up, so does Firefox, and then the antitrust issues begin, with maybe Chrome being spun into its own company or Android offering multiple engines. So they rather pay up, knowing that Mozilla will happily follow most of what they do.
I don't really blame them and I also don't think they gained their market share through illicit tactics. At least on the desktop, people switched because Firefox wasn't really a good browser until... 54 I guess?
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