WidevineCDM, due to an update implemented by Google (current owner of the module) in early January 2019, is currently BROKEN in official Basilisk 52.0.2019.02.11 [and in the unbranded fork Serpent 52.9.0, when used in Vista+; WidevineCDM CAN NOT be used on XP in a Mozilla type browser, due to it needing access to system (OS) decoders (h264/aac) via Windows Media Foundation (an OS feature of VistaSP2+)]. This is currently being tracked in UXP #962 ; I see no recent activity there, admittedly it's not a task for the faint-hearted to adapt mozilla60esr code (merged with lots of Google cr*p) to UXP's EME code...
Personally, I have little hope for a fix, but, who knows, maybe I'll be pleasantly surprised; if not, expect EME support to be probably also removed from official Basilisk, no wonder for "security" related issues . After all, no other app built on the platform currently (official Pale Moon 28, Iceweasel-UXP, BinOC's browser and e-mai client) has a need for EME/DRM...
As for Netflix: Supposedly, older installations who have acquired licence keys via the now deprecated WidevineCDM version (1.4.8.1088) used in Basilisk will continue to work in old Basilisk profiles, but new/first time Netflix users in current Basilisk will fail to get fresh lic (decryption) keys and thus will be unable to stream Netflix...
Netflix is one of the very few services that have a fallback mechanism to Silverlight NPAPI plugin on browsers not supporting EME (e.g. official Pale Moon), thus you can stream Netflix on PM28 with Silverlight 5 installed; PM28 has a native SSUAO for Netflix that forces it to fallback to Silverlight:
general.useragent.override.netflix.com;Mozilla/5.0 (Windows NT 6.1; WOW64; rv:45.9) Gecko/20100101 Firefox/45.9
95% of DRM'ed sites DON'T have the ability to use Silverlight (and Smooth Streaming) for their encrypted content and are broken in official Basilisk...
To stream Netflix in official Basilisk (or in the unbranded fork), one must disable EME in the browser, install and enable Silverlight 5, implement the SSUAO for Netflix and restart browser; hopefully then Netflix streaming in Basilisk should work...
Official Support Forum original thread: https://forum.palemoon.org/viewtopic.php?f=29&t=21500