I had weird things happen on Youtube when they switched to their HTML5 player, and found out it was because I had not re-approved fullscreen for Youtube; two grey buttons should show up at the top when you fullscreen, Allow and [Cancel?] with a checkbox for Remember My Choice.

Have you seen or clicked those? They can be hard to notice, especially on high-resolution monitors.

You've also checked 40.0 as your Firefox version, so you could try updating to 41.x to see if that solves the issue.

It's more then just youtube doing it too. Does it on twitch when I have it full-screened. Guess it doesn't know what auto update means so it started downloading soon as I click about.

Nope still doing it with 41.

Do you use the Status-4-Evar extension? If so, see: How to stop "transferring data from video" display on Twitch full screen?

Yep looks like that was it, new it had to be something with the bar cause it was showing up even when it's off