Firefox 81 has dropped the support for the
mozallowfullscreen attribute on the
<iframe> element. The unprefixed
allowfullscreen attribute has been available since Firefox 18.
According to the changelog,
mozallowfullscreen had actually been broken since Firefox 74 because newly implemented Feature Policy didn’t take it into account. The bug was fixed with Firefox 80, but the prefixed attribute is now gone in favour of the current standard.
To support older browsers, you still have to use the
allowfullscreen attribute together with