I do not know what impact if any ffmpeg has on widevine but my assumption is that ffmpeg is not to blame and its widevine that has shipped a broken version for Firefox. By default this plugin is automatically updated (Not by Solus but the browser itself) and is required for playing DRM protected content. Users on other distros are also reporting issues
Netflix and https://bitmovin.com/demos/drm do not detect a working widevine plugin on my desktop but it works fine on my laptop.
Desktop with widevine plugin auto updating enabled (Broken):
widevine version 4.10.2209.1 Last updated 9 April 2021
Laptop with widevine plugin auto updating disabled (Working):
widevine version 4.10.1582.2 Last updated 4 February 2021
opera-stable and vivaldi from the repository are working fine and using Widevine Version: 4.10.2209.0 and when you try and force them to check for newer widevine from chrome://components/ it does not detect a newer version being available.
EDIT:
New ffmpeg does not help: https://dev.getsol.us/R742:93eb02b718d44d957b7e4be5764ce2e35f95db37