If money wasn't a consideration perhaps ddg become a good partner. That combo could shift as much as 5% of google traffic away from search.
Google pays Apple a reported $8 billion a year to be the default search engine for iOS. It pays Mozilla less than $600 million (https://www.cnet.com/news/google-firefox-search-deal-gives-m...).
Doesn’t that tell you how little Firefox traffic is worth?
As for iOS, this source says there were 900 million iPhone users (doesn't include iPads) as of Jan 2019: https://www.idownloadblog.com/2019/01/30/900-million-iphones... , and this one says 1.5 billion Apple devices (probably includes Macs) as of Q1 2020: https://9to5mac.com/2020/01/28/apple-hits-1-5-billion-active...
I'll arbitrarily guess 1.2 billion iOS users (the rest being Macs). In that case, there are maybe 6x as many iOS users as Firefox users. Therefore, if Google paid the same per Firefox user as they did per iOS user, they would have paid $1.3 billion for Firefox users. That's a difference of about 2x per user from what they actually paid.
iOS users have demonstrably paid for (or been gifted by people who paid for) premium electronic devices, which I would expect makes them more valuable advertising targets. I'm mildly surprised that the difference isn't larger, but I don't have a good feel for advertising expenditures.
Google could kill Firefox by just not renewing its search contract.