Some videos won't play in background
Firefox for Android can continue playing video even if you switch to another tab or app. However, sites can independently detect if it is not the active tab and choose to pause the video rather than playing it in the background, and some popular video websites have done so. We are continuing to improve Firefox for Android to better meet the demands of our users and you can continue to use this feature on other video websites.
How does this happen?
When a webpage moves to another tab, the Page Visibility API triggers a visibilitychange event regarding the visibility of the page. Some website developers may configure their website to detect these events, and pause video playback. Other than Page Visibility API, websites may choose to pause video playback when the user leaves its full screen player, by detecting a fullscreenchange event triggered by the Fullscreen API.
How can I fix this?
The behavior described is normal and expected, when a website has chosen to use the visibilitychange event or fullscreenchange event in this way. However, the behavior can be disabled by preventing the normal events from triggering.
As an example, an extension can listen for and intercept visibilitychange and fullscreenchange events so a website will not be able to react to a tab being hidden.
Please visit Video Background Play Fix to see or install an example of such an extension.