oh god. I thought every social media was breaking my audio when uploaded, but no, they're transcoding my mp4 into a perfectly sync'd mp4.

which firefox/their video player then plays out of sync.

I feel like less shit for making a video that played wrong, I thought I fucked up the sync.

but now I have zero idea how to fix this

yeah, it's not tumblr that's fucking this, I made my own HTML that was just <title>VIDEO</title><video width="320" height="240" controls><source src="video.mp4" type="video/mp4"></video> and it still plays out of sync in firefox
@foone one thing that usually helps is to run video through handbrake and use vlc to view to check. then host it somewhere to check via browser