It could be that the audio stream and the video stream are separate files instead of in one file and sync gets lost if you mute the audio.

I am having this issue with videos on Twitter, which start muted. The audio is maybe two seconds out of sync with the video.

Using Firefox 106.0.2 on macOS Ventura

It could be that the audio stream and the video stream are separate files instead of in one file and sync gets lost if you mute the audio.

The problem happens on live streaming as well as posted videos. I've tried starting Firefox in "safe mode", and it still happens, so it's not due to any add on.

Lately using Firefox, the audio is not syncing with video on Xfinity streaming. The audio seems to precede the video by about a half second or so. When I first load a video, it works properly, but I finally discovered that the delay occurs only after I mute the audio using the "PLAYING/MUTE TAB" button. Before muting, audio is in sync with video. But when I use it once to mute, after un-muting the audio is out of sync. The only way to get the audio back in sync with video is to reload the video source. This does not happen when viewing YouTube, nor does it happen with Xfinity when using the Chrome browser. I first noticed this about 2 months ago. Before that, I had been using Firefox for years to watch MSNBC in a minimized window, muting during commercials. Unmuting used to cause no problems. I'm wondering if this problem is unique to something in my installation or if it is a Firefox problem.

Update: I was wrong about YouTube. Audio DOES NOT sync on YouTube videos after muting and then un-muting.