With focus on the volume slider, left/right adjust it, but also jump back/forward 10s.

[mozjag]

I noticed this when I was jumping back a bit in today's F2 stream and the sound got quieter and quieter until it as muted.

Steps:

1. play a video stream
2. unmute by clicking somewhere on the volume slider
3. hit left a few times to go back (alt+l first as needed)

Notice both the volume going down and the playback rewinding by 10s.

Suggested fix: change it so left/right only go back/forward, and up/down adjust volume, regardless of where the focus is.

Note that up/down for volume currently don't work when focus isn't on the volume slider, so this is a bit of a two-fer.