Alice Averlong🏳️⚧️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
Raptor 
@foone what codec? firefox uses your system libs for playback, you could have something failing to load in libva/libvaapi and falling back to software which if it can't keep up can lag behind during playback.
@raptor85 looks like handbrake is outputting h264 mpeg-4 AVC part 10 "avc1", with AAC audio at 48khz