FWIW, my take back in 2010-2015 when I was at Moz wasn’t quite that cynical. I’m sure that was at least some of it, at least after Chrome started really gaining users, but Mozilla and Google had a good relationship for a long time before Chrome was popular.
Mozilla used to actually have their offices on Google campus (even though I started after the HQ had moved out, I was still given a Google badge for their cafeteria) and I believe the early Chrome team already had ex-Moz people on it. Goals around standards, etc, were relatively aligned, at least early on.
When Mozilla decided to go rapid release, Google was the one who walked them through how to do it (then Mozilla bungled it by diving into a very accelerated schedule and not considering how it affected add-ons, but that’s another story). It was all pretty friendly.
I’m also pretty sure Bing offered similar incentives for referrals, just without the flat-fee contract Moz had with Google, and they didn’t have the same antitrust issue in that particular space. Referrals and default search provider status have always been part of the financial model between browsers and search engines. Dealing that value back to themselves is one of the reasons search providers publish browsers.
I was pretty surprised when there was such a huge backlash against Apple for that recently since it had been SOP in the business for a long time.
To your point though, given the current market share of Firefox, $500M has to be about more than that if that’s an accurate number.