When streaming video containing adds, if I leave the tab or FF, the adds pause. They never pause the movie, just an add. Creepy to me because I want to do something else while the add is playing.
Would it be possible for a plugin to hide the fact that one has switched to another tab?
First of all - it’s “ad/ads”, not “add/adds” 
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This likely happens because Firefox is blocking auto-play. So it will pause whatever is trying to play automatically, which is commonly an ad.
I guess the most obvious solution is using a good adblock, for example:
There are a number of reasons why playback might pause when the page is unfocused. The page could be monitoring the page’s focus or blur events, it could be monitoring visibilitychange events, or they could do something hackier like using a chain of requestAnimationFrame callbacks to detect when frame rendering is throttled.
It’s possible to shim these API surfaces to make it more difficult for sites to detect focus changes, but doing so would require programming experience (preferably web development) and some knowledge of browser extension APIs to implement.