Do you have any privacy/ad-blocking extensions installed? I used to have the same issue and realized it was due to the DuckDuckGo Privacy Essentials extension. When I turn that extension's protections off for cs.opensource.google then it works.
Thanks so much for your time, and for sharing the issue.
Back when it was happening (maybe ~1 year ago?), I was using the latest versions of Firefox and DDG Privacy Essentials and it ocurred even if I went directly to https://cs.opensource.google. I had confirmed back then that when I turned DDG Privacy Essentials on I got "Permission denied", but with it off I was able to view the page.
I'm on the latest verisons of Firefox and DDG Privacy Essentials now. Seems it is no longer an issue in the latest version(s).
What doesn't work?
Having those links to quickly take a look at the implementation is what made this documentation so good.
2. In that case, why allow reading the code un the browser ?