Firefox suddenly defaults to flash again on youtube

[Heinz Diehl]

Hi,

the subject says it all. It came out of nowhere, the only thing I
remember is having used youtube-dl on this machine a singe time
today. Firefox used the HTML5 player on youtube, now flash is back as
the default player, and nothing helps to get rid of it. No downgrade,
no upgrade, only the manual switch to HTML5 on the youtube config
page. Next time, flash is back as the default player.

However, after creating a new user, the default player is html5 for
this one.

Where is the config option, switch or whatever changes the default
player in firefox back to html5 again?

Thanks,
 Heinz