Firefox supports the widevine drm since version 47 on Mac OS X and Windows. The Linux version can utilize the chromium widevine .so file with a workaround: https://github.com/i-rinat/gmp-widevine/blob/master/how-to-enable-on-firefox-47.md Please add a (masked?) useflag to add "ac_add_options --enable-eme=widevine" to .mozconfig, so users can enable widevine without having to patch the ebuild.
If you feel I have closed your bug and it is still a current issue, please reopen and update it completely. We will not work bugs that have no ebuild in tree any longer or can not be reproduced with a current system. Thank You for your support and understanding The Mozilla Team