I can't use Google Meet on firefox/zen, I tried every setting combination I could find but the video call quality is still not comparable to chromium based browsers, so at work I reluctantly switched to Vivaldi.
Have you tried switching your User Agent (with or without a helper extension) to Chrome/Chromium/Edge to see if that makes a difference? I have heard that some G sites that are clunky or broken seem to work better under Firefox when it identifies itself as Chrome.
I think it's widely speculated that Google sabotages how their own products work in Firefox. I don't know if there is actual evidence to support that though.
I'm in a Zoom shop now, but when I was in a Meet/Hangouts shop, I used Chrome for that, and Firefox for everything else. If you're on a Mac, the utility Choosy can send links to appropriate browsers based on patterns.
If you figure this out please let me know!