Original report:
Bug 1560896 - Cloudflare protected websites do not load properly with Tor proxy
Caused by:
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.
Well from my point of view that’s something to fix for Mozilla. Sites behind Cloudflare are not the only ones using webp. And webp is no only used for the captcha page. 
I cant reproduce that either. I got the challenge page for both of the mentioned examples.
I generally find the bug report a bit confusing. If the challenge page is supposed to show up but does not, it would either mean the assumption is incorrect or there is an issue with Cloudflare. Without being able to reproduce it, my guess would be the former to be honest.