You can toggle DRM off/on to see if that makes Firefox (re)load DRM components.

• Settings -> General: Digital Rights Management (DRM) Content
• https://support.mozilla.org/en-US/kb/enable-drm

You can also check the connection settings.

• Settings -> General -> Network: Connection -> Settings

If you do not need to use a proxy to connect to internet, select "No Proxy" if "Use the system proxy settings" or one of the others do not work properly.

See "Firefox connection settings":

• https://support.mozilla.org/en-US/kb/websites-dont-load-troubleshoot-and-fix-errors

Does it still happen in Troubleshoot Mode?