If the top search engine spot in Firefox would be so valuable, than there would be real bidding wars. $400M/year is nothing when it comes to BigTech. Frankly, anyone could pitch a business plan for an investment bank to get a few billions and outbid Google. If anyone cared... but being the default search engine in a browser that virtually nobody uses isn't worth a lot. But it's a fantastic decoy, if you have nothing better.
(Also, Yahoo offered more when both Firefox and Yahoo still mattered. Which is not the case anymore. The only viable search engine today is Bing, who stopped caring about search, as AI seems to be more lucrative for them. Marginalia is also here, but that guy has less money than MS, prolly)
Safari exists exclusively on iOS and MacOS. On Android, Windows and Linux Chrome has virtually 100% market share. Do you know why Google spends more money on Safari? Because on that platform they want to be the default browser. Safari doesn't depend on Google at all. Not like Firefox.
Firefox dies in 2 minutes once Google decides that it has outlived their usefulness, at which point all their users default to Chrome, where they don't have to pay to be the default search engine. And at the point they would have to pay only Safari, without any negative impact on their search traffic. But as it stands today, they would have no competitor on the vast majority of the consumer computing systems.