Firefox currently downloads the (Google Widevine) DRM blob automatically. Firefox will prompt you before using the DRM blob to play video, but you have two options if you don't want it on your computer:

* You can uncheck Firefox's "Play DRM-controlled content" option (which will delete the DRM blob from disk). https://support.mozilla.org/kb/enable-drm

* Or you can install one of Mozilla's "EME-free" builds of Firefox. The only difference between regular and "EME-free" builds is the default value of the "Play DRM-controlled content" option.

http://archive.mozilla.org/pub/firefox/releases/62.0.2/