Choose a meaningful default
Mozilla’s current help lists Block Audio, Block Audio and Video, and Allow Audio and Video. Inspect the default in the Autoplay settings under Permissions, using the settings search if the page layout differs. Record the current choice before changing it.
An illustrative preference is to prevent surprise noise while permitting silent previews on ordinary sites. Another viewer may prefer no automatic motion at all. Neither preference should be assumed from the word autoplay alone. Use the option whose stated scope matches the actual need.
Check the website’s exception
A specific site can have an autoplay permission that differs from the default. Mozilla documents the address-bar permission control and a saved exceptions list. Inspect the intended domain and avoid broadly allowing every website merely to make one familiar player behave differently.
After a deliberate change, reload or reopen the relevant page as appropriate and compare the same action. Note whether you are visiting a new page, selecting Play or advancing to another item inside an already active player. Those are different interactions and should not be collapsed into one observation.
Keep manual viewing separate
An autoplay preference concerns automatic starting behaviour, not a promise that the media itself is compatible or authorised. Check whether a deliberate Play action works. If playback still fails, preserve the player message instead of repeatedly changing a preference that no longer matches the problem.
In a support note, include Firefox version, selected default, saved exception and the exact action that starts or fails to start video. Keep account details out of screenshots. Restore the prior preference if the experiment does not help, so a single viewing test does not silently change the behaviour of all future sites.