It's probably more on the lines of Google losing advertising share to every other company (Meta, Amazon, Unity, Microsoft) that has gotten into the ad business in recent years - all with minimal experience in ads, but either data, infrastructure, or visitors to sell. Mozilla definitely will have the infrastructure and visitors, even if opt-in.

I don't agree that they'll overtake Google, or could have overtaken Chrome with their product tie-ins/offerings. Google is a beast, whereas the average person probably couldn't tell you who makes Firefox (or maybe even what Firefox is).