That and to keep the open source community pacified. If Firefox wasn't around it's very likely that the community will develop something much better which may be completely out of control for corporations.
How do you figure? 'the community' isn't very interested in ergonomics or onboarding, see Linux distros.
Well, they did fund Firefox since long before chrome. Maybe the reason is the same as why they "fund" Safari.
Then why do they pay Apple far more money for the same service?
I suspect there are far more iPhone Safari users than firefox users overall.
Well yeah, that's why Apple gets a much bigger check. But they still buy the same thing on Safari where it's hard to claim it's for an eventual antitrust case.