I am working on some forks of Firefox, and one of them won't play certain videos protected by widevine. The browsers seem to have the same version of widevine installed (I even sha256sum'ed the files and they seem to be identical). The two browsers are forked off FF 102 (one is .8.0 and the other is .9.0), but they have various commits on top of those.

So, I'm trying to understand the problem and, ultimately, fix it.

I've searched this site for various keywords and it looks like there are various discussions that are relevant. However, the most relevant seem to be specific to *downloading* the videos, but I'm simply trying to play them. Is it still the best place to ask?

Looking at the network traffic in the browser dev tools, it seems like the browser makes a request to a license server, and the relevant server is responding with a 4** on the browser that fails, but 200 on the browser that works. I compared the HAR for the request and they look identical, apart from the actual request payload - so I presume that the payload is being generated incorrectly, or from assets/settings that are not correct (or potentially a difference in the build process).
So, I want to understand how that payload is generated.

So, I'd appreciate a pointer in the right direction.

A video that works and claims to be protected with widevine (above the video it says "Detected (FF), using Widevin"):

https://bitmovin.com/demos/drm

One that doesn't work (the one named 'Protected . MPEG-DASH Multi-key DRM'):

https://demo.castlabs.com/#/player/demo

The error it reports is lengthy, but the beginning is:

Code:
FATAL Clpp-Error [Category 6 - Code 6007]: Error while fetching license! Caused by FATAL Clpp-Error [Category 1 - Code 1001]
about:addons says Widevine is 4.10.2557.0 and I'm working on an Apple Mac 13.2.1 (arm).

Thanks in advance for any help.