I finally solved a mystery that I hadn't been able to figure out for years: why Tor Browser doesn't get the "Attention Required!" CAPTCHA page and vanilla Firefox does.
Turns out it's browser fingerprinting that determines whether you get that page. I just installed the Privacy Settings browser on Firefox. One of its settings is "websites.resistFingerprinting" which is set to True in Tor Browser and False in vanilla Firefox. Setting it to True avoids the CAPTCHA page on Cloudflare websites.
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