Hi, we've heard on the past about webm video support on Flash Platform. Is this still being considered?
I think it's safe to say at this point that we're not going to add support for additional codecs.
Although what jeromiec83223024 says will apply to the Flayer Player Plugin, and that's probably what you were interested in, you can use Adobe Animate to make something for the web that plays webm video.
Start a new HTML5 Canvas FLA, add a Video component, and set its source to be a webm video. It will work in Chrome and Firefox.