Having Firefox around for just a few hundred million per year is convenient for Google to not have Chrome considered a monopoly (although Edge getting back the market somehow changes it). Anyway it feels like it would be stupid and potentially more costly for them to do moves to sabotage Firefox. It's like an insurance.
But the big question is if there is an agreement or "understanding" or something that Google only pays as long as Firefox doesn't become a threat, i.e.:
- don't fix the plugin ecosystem
- don't fix theming
- don't get too far ahead technically (i.e. a good reason to get rid of the servo team)
- don't claim too much authority (i.e. get rid of MDN)
Of course this is wild speculation and mostly in jest.
