Google has a dominant position in the advertising industry with AdSense and their other advertising-related products. Google also has a dominant position in the browser market with Chrome. Google can’t use that dominant position in the browser industry to make changes to their browser that would negatively affect their competitors in the advertising industry without consulting competition authorities which are trying to make sure they aren’t intentionally harming their competition in the ad market using their dominance in another market (the browser market) to benefit themselves. Firefox is small enough (and generally doesn’t have any other services they could leverage) that they can just make changes to their browser without running afoul of any competition concerns.

There’s also the advantage that Google has when it comes to the large number of popular first party services they have, like Gmail, Search, YouTube, etc. Using those services alone, they may be able to develop a profile of a user that’s better than the competition would be able to do with the new Topics API, Protected Audience API, etc and thus even just getting rid of third-party cookies without a replacement might be seen as anti-competitive. This is probably why places like the EU are also forcing services to make it possible to unlink those services and not have the data shared between them.