Bug 1560896 - Cloudflare protected websites do not load properly with Tor proxy
Bug 1417463 - consider changing default accept header to */* (arrived in Firefox 66)
Current open bug that would fix issue:
Bug 1544231 - Missing ‘image/webp’ in default value of the ‘Accept’ header
It would be great if someone from Cloudflare could leave some feedback in the above open bug on the need case for the “image/webp” default accept header for showing the captcha challenge page.