Ideally, in the LibreWolf browser configuration screens there would be some window or admin screen with "media.gmp.decoder" (for example) as a listed configurable option. Possibly more generic: "Decoder Options".
And within that some buttons or check boxes labeled True or False, On/Off, Enabled/Disabled etc.. shown alongside.
Including some explanation about what each choice actually means and does. May include additional information about how the choice interacts with or may not be available with respect to other choices and options.
Unfortunately, such configuration guidance is either not done at all, done poorly, and often without any helpful explanation.
If the answers are not provided via the browser documentation or otherwise found on Librewolf's website via Forums and/or FAQs then contact LibreWolf Support.
Are you able to capture some screenshots of the settings and choices (values, available increments, and so forth) that you have listed? Where you see the choice and whatever options are available?
If so, do so and post the screenshots here using imgur (
www.imgur.com).
Someone may recognize the settings and be able to provide applicable answers thereafter.
I am not sure what else to suggest.