After updating to the lastest version of firefox (48.0.1), something strange happened - While I'm listening music with Windows Media player, and when firefox is playing audio/sound/music (for example when I'm receiving a message on facebook, or watching videos on YouTube) WMP is paused. But when the audio coming from firefox is finished playing, the music from WPM continues to play. It could be a nice feature if we talk about watching videos, but It's extremely annoying when I receive messages that outputs short sounds (It just pauses my music for a second and then continues again).

I couldn't find any reason or option to disable it. Tried to google it but didn't find anything. I hope I wrote it well, I'm still practicing on my english...