Media Recorder ondataavailable not fired in Fire Fox

Heres my code to record the screen (with audio)

  let options = {
        video: {
            cursor: true,
            displaySurface: 'window', // monitor, window, application, browser
        },
        audio: true,
    }

    voiceStream = null;
    desktopStream = await navigator.mediaDevices.getDisplayMedia(options);

    if (audio === true) {
        voiceStream = await navigator.mediaDevices.getUserMedia({video: false, audio: true});
    }

    const tracks = [
            ...desktopStream.getVideoTracks(),
        ...utilVideo.mergeAudioStreams(desktopStream, voiceStream)
    ];

    stream = new MediaStream(tracks);
    console.log('Stream', stream)
    blobs = [];

    rec = new MediaRecorder(stream, {mimeType: 'video/'+fileFormat+'; codecs=vp8,opus'});
  
    // If our stream goes inactive (user clicked "Stop" on the browser bar), stop it
    stream.addEventListener('inactive', () => {
       console.log("inactive");

    })

    stream.getVideoTracks()[0].onended = function () {
        console.log("on ended");
    };

    rec.onstop = function(e) {
        console.log("onstop");
    };

    rec.ondataavailable = function(e) {
        console.log("data available");
        blobs.push(e.data);
        downloadVideo();
    }



stopCapture() {

        console.log("stopCapture");
     
       if(rec.state!="inactive"){
            rec.stop();
       }
        if(stream!=null){
            stream.getTracks().forEach(s => s.stop())
            stream = null;
        }
    }

downloadVideo(){
    console.log("download video");
    blob = new Blob(blobs, {type: 'video/webm'});
    let url = window.URL.createObjectURL(blob);

    let downloadLink = document.createElement('a');
    downloadLink.href = url;
    downloadLink.download = activeUser.activeProject.getName()+'.webm';

    document.body.appendChild(downloadLink);
    downloadLink.click();
    document.body.removeChild(downloadLink);
}

I call the stop function when the user presses escape. This works fine in Chrome, and I get a download of the file, but nothing happens in FireFox and I can see it’s never getting into the ondataavailable event.

Using the console logs in Chrome when I press escape I get the "stop capture", "Stream" and then the stream, "data available", "download video"

In firefox if i look at the console I get "stop capture", "Stream" and then the stream. Nothing else

Can anyone suggest why the ondataavialable is not called the same was in Firefox as it is in Chrome?