[pteraspidomorph]

According to caniuse some headers are necessary for it to work on Firefox. I guess the developer has to fix the demo.

[JeromeWu]

In fact, the header must be set in the server-side, which means I cannot do that as I am using github pages. You can check more details here: https://github.com/ffmpegwasm/ffmpeg.wasm/issues/102

[pteraspidomorph]

I noticed you were using github and completely failed to think of that! My bad.