I think it is now auto-configured to detect if it is installed. Or at least that was supposed to be one of the features of this release.
https://www.phoronix.com/scan.php?page=news_item&px=Firefox-...
https://www.phoronix.com/scan.php?page=news_item&px=Firefox-...
https://support.mozilla.org/en-US/kb/enable-drm