Browser HTLM5 player on YT - Line through speaker icon but not mute

I can't remember if I've seen this before but when watching YT videos a couple of days ago I became aware of a diagonal line through the speaker icon to the bottom left in the player. The sound isn't muted but when I do mute the sound a pixel above the line in white parts of the graphic disappears, as shown below. Notice how it is still showing the "sound waves" when muted also.

Using FireFox 50.1.0.

Any ideas?


Not Muted



Muted

This seemed to resolve itself but...

Great. Now I have a black player until I refresh. This happens very regularly.

Is it time to ditch FF?