They can always test it out against all their ads and make sure it works, putting out a fix if it ever doesn’t.
They could do this even if they weren’t funding mozilla. Ad’s aren’t exactly reliant on bleeding edge web standards anyway. You’re thinking about tracking tech, which they don’t have any input in for firefox.
We’ve also seen recently how they’re trying to make it so people can’t run ad blockers
Well yes, and mozilla was quite vocal in their opposition, demonstrating that Google doesn’t have much control over them.