[matroska,webm @ 0xdf7650] Format matroska,webm detected only with low score of 1, misdetection possible!

[L4z3x]

i got the error when trying To concatenate multiple webm videos
after writing all the files i execute this:

await ffmpeg.writeFile(listFile, fileListContent);
  await ffmpeg.exec([
    "-f",
    "concat",
    "-safe",
    "0",
    "-i",
    listFile,
    "-c",
    "copy",
    "exam-unwrapped.mp4",
  ]);

i tried this on my machine and it was working, but when i try in firefox , it doesn't work and i get these in the log
[matroska,webm @ 0xdf7650] Format matroska,webm detected only with low score of 1, misdetection possible!
Truncating packet of size 12131 to 685
[matroska,webm @ 0xdf7650] EBML header parsing failed
[concat @ 0xdee9f0] Impossible to open 'opening.webm'
file_list: Invalid data found when processing input
Aborted()
note that the videos were in mp4 format then i converted them with this command:
ffmpeg -i "input.mp4" -f webm -c:v libvpx-vp9 -c:a libopus "output.webm"
thanks in advance