To convert your output.mp4 file into a .webm file using FFmpeg, you can use the following command:

```
ffmpeg -i output.mp4 -c:v libvpx -c:a libvorbis output.webm
```

In this command, "-i" specifies the input file name, "output.mp4". "-c:v" and "-c:a" specify the video and audio codecs to use for encoding the output file, respectively. In this case, we are using the VP8 video codec and the Vorbis audio codec, which are commonly used in WebM files. Finally, "output.webm" specifies the output file name.

This command uses the libvpx and libvorbis codecs, which are open source and provide good quality and compression for WebM files. It is a simple and efficient way to convert your file from MP4 to WebM format.
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