If you get the error message “Firefox is installing components needed to play audio or video on this page. Please try again later.” or are constantly asked to enable DRM each time you visit the Plex Web player, you can follow these steps to eliminate the issue:
Go to about:config in the address bar.
Ensure that media.gmp-widevinecdm.visible and media.gmp-widevinecdm.enabled are false. Double click the items to toggle them.
Please note that this will break DRM playback on your computer. (This affects most major streaming services, including Netflix, Hulu, and Amazon Prime Video.) That being said, if you had this problem, DRM was probably already broken or disabled.
Plex: This issue can be mitigated by not requesting DRM on page load, but instead requesting it later when the user actually opens a video that requires it. (Presumably for the new ad-supported movie streams that are being added.)