It's improved a bit, and actually works fine in Firefox Nightly right now, but you have to craft the web client URL directly. The UI will try its very best to make you download the client.
Seen somewhere else: when you get to the web page that launches the app, don't allow that launch, hit (IIRC) "Retry", still don't allow it, and the page should say "Having trouble?" offer a link to the web version.
Or better yet, don't use Zoom...