Larger companies such as Apple and Google have to worry somewhat about security and privacy related issues. As part of that, they can't let you show a web page that plays sound without the user asking for it. The accepted way to know that the user wants sound is that you require them to click, or touch, before the animation with the sound can play with sound. Firefox is open source I think, and so they can get away with doing what is technically possible, without worrying about privacy issues.
The solution is to have a button that has to be clicked or touched before the animation plays. That should work in all cases.
Something else to know, the most recent versions of Animate allow Stream sync for sound in HTML5 Canvas. You should be able to get better lip sync with Stream instead of Event sound.