This is related to the new integration with media keys on the keyboard. If you aren't interested in using media keys to manage audio in Firefox, you can disable the integration this way:

(1) In a new tab, type or paste about:config in the address bar and press Enter/Return. Click the button accepting the risk.

(2) In the search box in the page, type or paste mediakeys and pause while the list is filtered

(3) Double-click the media.hardwaremediakeys.enabled preference to switch the value from true to false

You might need to exit Firefox and restart it to break the connection with Windows, or perhaps closing all tabs that have played audio will be enough, not sure.

Thank you for solving my problem so fast.. Kudos to you and Firefox. J B