I don't think there's anything going on here explicitly targeting Firefox. Adblock/UMatrix/etc are doing their job, blocking cookies and other tracking and heuristics that allow sites to decide you are "likely human" and skip showing you a captcha. Using Chrome (and being logged in with your account connected to Chrome) is providing a heuristic that Firefox cannot, allowing you to skip captcha since you are being tracked by the browser itself. So it's more implicit than explicit.