circa 2016, there used to be an option:
https://www.trishtech.com/2016/12/how-to-completely-disable-drm-videos-in-mozilla-firefox/
Discussion
i'd add to that, probably widevine DRM can be exploited in some way that we can't know because closed sauce.
i'm just using https://webextension.org/listing/block-site.html and just blocking any site that causes mozilla to vomit a DRM snackbar at the top of the page from now on.
it happens on spotify previews, this i have confirmed, and open.spotify.com is on my block list permanently now.