Firefox supports the WebCodecs API behind a feature flag. Set dom.media.webcodecs.enabled to true. I am able to use your app this way -- the media import, timeline, and player functionality seems to work fine.
We're aiming to release this half, so in a month or so, on all desktop platforms, mobile will follow shortly after.
Then we'll gradually optimize (e.g. enable the use of more hardware encoders, decoding being generally in hardware at launch of supported) release after release, but generally almost everything will be supported at launch.
