You can control playback on Prime Video on a computer by using your keyboard.
The keyboard shortcuts that you can use while watching Prime Video on a web browser are:
SPACE to toggle play/pause.
F to enter or exit full screen.
Esc to exit full screen or exit playback.
Left arrow to rewind 10 seconds.
Right arrow to fast-forward 10 seconds.
Up arrow to increase volume.
Down arrow to decrease volume.
M to toggle mute.
C to toggle subtitles/captions on and off, and to switch between available subtitle/caption languages.