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.