They're getting garbage in with the service too - about the last dozen (at least) or more times I've had to do it you can clearly see that one of the options is what the AI thinks should be the answer, but isn't. But if you don't select it before clicking 'verify' it gives you a little 'please select all taxis' or whatever.
No AI, that's just a car, not a taxi.
But who cares, I just click anyway and feed bullshit in and in a few years it'll think everything is a taxi.
It is not about Chrome vs Firefox. It is about how much information they can get about you.
The idea behind the "I am not a robot" captcha is that if you act like a human, then you don't need to be shown pictures for further confirmation. The more you allow tracking, the more info they have to figure it out.
A big one is being logged in to your Google account. Having access to all your history really helps. And because Chrome heavily encourage you to setup a Google account, Chrome users are more likely to be logged in than Firefox (or other) users.
Spoofing your user-agent is likely to be worse. If you have a Chrome user-agent but your browser don't act like Chrome, that's a big red flag. Bots often spoof their user agents.