ffmpeg -i input.webm -c:v libx264 -preset slow -crf 22 -c:a aac -b:a 128k output.mp4
This ffmpeg command converts a .webm video file to a standard .mp4 file using the libx264 codec for video, aac codec for audio, and a CRF value of 22. The preset is set to 'slow' for higher quality encoding, and the audio bitrate is set to 128 kbps.
note that there must not be spaces in the name of the .webm file to be converted. it causes the command to fail.