User Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:57.0) Gecko/20100101 Firefox/57.0 Build ID: 20170903220032 Steps to reproduce: 1. Visit https://www.youtube.com/ 2. Click on a few videos with the middle mouse button, or right-click and select "Open Link in New Tab", to open a few tabs with videos. Don't visit (click on) the new tabs! 3. Notice the "Play" icon in the tab, which means a video is ready to be played 4. Hover your mouse cursor over the tabs Actual results: All videos in tabs that were hovered over start to play Expected results: None of the videos should have played until you actually visit (click on) the tab