Start Firefox quantum and type this command in the address bar

 about:config

Search for this string :

browser.tabs.remote.autostart.2 


and Change it to : browser.tabs.remote.autostart.2=False

do the same for those too
security.sandbox.content.level = 3 (standard)

browser.tabs.remote.autostart = false (standard)  

See this link for more details :

https://forums.sandboxie.com/phpBB3/viewtopic.php?f=11&t=24937&p=130963&hilit=no+sound+youtube#p130963