Some users are updating Firefox but failing to update Widevine. We have seen evidence for this in the new telemetry prefs we are tracking. media.gmp-widevinecdm.lastDownloadFailReason:

Error: Failed downloading via ServiceRequest, status: 0, channelStatus: 2153390067, errorCode: 2, reason: error

2153390067, or 0x805A1FF3, corresponds to SEC_ERROR_UNKNOWN_ISSUER. This suggests these users have some antivirus program that is interfering with Widevine updates. Perhaps it is intercepting only certain domains (e.g. https://redirector.gvt1.com/) because we appear to update Firefox just fine for many of these users.