I wanted to comment my experience with FF45.b8, If I have open thunderbird38.8.0, and I try to start a video on youtube, firefox is blocked, with ctrl esc, I can close it.
But if the thunderbird is closed, I have no problem seeing it.
Sound does not work with arca50,for me.
The thunderbird, open, also caused me a lot of screen problems with the Vbox.
Saludos