Tabs are considered backgrounded sooner than in Fennec #291
Description
Very noticeably with Youtube, where without an add-on (#237) (or possibly a subscription), playback on the mobile page stops as soon as Youtube considers the page to be in background.
With Fennec this only happens when you actually switch to a different tab, or else the whole app is in background. With the reference browser, opening the settings menu or merely the tabs tray is already enough to trigger this behaviour.