Try this
The same video, re-encoded. (208kB) - so 1/10th the size
ffmpeg -i scr1.mp4 -vf "scale=720:-2" -c:v libx264 -crf 23 -preset medium -c:a aac -b:a 128k scr2.mp4
ffmpeg -i scr2.mp4 -movflags faststart -c copy scr3.mp4
Upload this to your firebase storage and test (Try the native video player)
Note: you can do all this encoding on the device using @vknow360 's excellent ffmpeg extension:
EDIT
This 2 pass command creates a useable 91kB video from the original:
ffmpeg -y -i scr1.mp4 -vf "scale=720:-2" -c:v libx264 -b:v 50k -preset veryslow -c:a aac -an -pass 1 -f null /dev/null && ffmpeg -y -i scr1.mp4 -vf "scale=720:-2" -c:v libx264 -b:v 50k -preset veryslow -c:a aac -b:a 128k -pass 2 scr2a.mp4