That's not good. Does it matter whether you pause the video before closing the tab? Are there particular sites where you see this happening? Assuming it's "family friendly" feel free to post a link.

Some video-related recorders/downloaders may modify how video sites work. If you have any of those, try disabling them to see whether that makes any difference. You can view, disable, and often remove unwanted or unknown extensions on the Add-ons page. Either:

• Ctrl+Shift+a (Mac: Command+Shift+a)
• "3-bar" menu button (or Tools menu) > Add-ons

In the left column, click Extensions. Then cast a critical eye over the list on the right side. All extensions are optional. If in doubt, disable.

Often a link will appear above at least one disabled extension to restart Firefox. You can complete your work on the tab and click one of the links as the last step.

Any difference?

tried with all the extensions disabled, there are only few of them, pausing the video before closing the tab wont make a difference, im using firefox 64 53, the problem is even tab is closed firefox still continue to consume bandwidth, only way to stop is closing firefox.

i dont know by any means this sites are legal but i only stream video from http://kissanime.ru and http://kissasian.com, youtube is not affected