There are several patches under its patches source directory, and there are different sort of packages, one example is the sed patch to avoid including pocket in the build. The DRM widevine is not included either on the build, though it can be installed if you want it installed (probably there’s a patch for that somewhere).
But I no longer see removing binary blobs being advertised by Librewolf, it’s been a while since I don’t check on their site…