A simple shell script to record the contents of a window to a video file using only ffmpeg

screencast.sh

#!/bin/sh

outfile="${1-screencast.mp4}"
echo -n 'Click on the window you wish to record...'
tmpfile=$(mktemp /tmp/screencast.XXXXXX)
trap "rm -f $tmpfile" 0 1 3 15
xwininfo > $tmpfile
echo 'done.'

origin=$(awk '/Corners:/ { print $2 }' $tmpfile | sed 's/\(+[[:digit:]]*\)+/\1,/')
width=$(awk '/Width:/ { print $2 }' $tmpfile)
height=$(awk '/Height:/ { print $2 }' $tmpfile)
rm -f $tmpfile
trap - 0 1 3 15

echo "Recording to ${outfile}..."
ffmpeg -video_size "${width}x${height}" -framerate 24 -f x11grab -i ":0${origin}" -preset ultrafast -qp 0 -y "$outfile"

Apparently both Android and Firefox object to native 4:4:4 chroma format that the x11grab driver generates; you'll probably want to postprocess the video anyway for better compression, so add -pix_fmt yuv420p if compatibility matters to you.