It’s a DRM system called Widevine, that’s currently maintained by Google. It ships in Chromium/Chrome and Firefox as a closed-source binary blob. Firefox asks you before running it, since you may not want to run proprietary code.

You can tell Firefox not to run it, or disable it in the plugins. Instructions are here: https://support.mozilla.org/en-US/kb/enable-drm#w_disable-the-google-widevine-cdm-without-uninstalling. Regular videos will still play properly, but videos with DRM will fail to play. Note that practically every paid streaming service uses Widevine DRM, so if you disable it, none of them will work any more.

Plex are likely using it for their free streaming content. I’d guess that they’ve licensed it only for streaming, and need to enforce that users can’t download or record it.

Your own Plex content does not use DRM, so if you’re only using Plex for your own content, it’s fine to deny Widevine from running.