Does it do that on other Browsers?

I have never had that feature in Firefox. The videos have always kept playing in the unfocussed tabs after switching to a different tab.

That feature may have been provided by an add-on for Firefox, which I have never used - if such ever existed.

the-edmeister said

That feature may have been provided by an add-on for Firefox, which I have never used - if such ever existed.

Right you are. I mis-remember. "Play/pause" and Mute All Inactive Tabs kinda get the job done.

Firefox recently incorporated a feature to throttle media streaming in background tabs to improve performance in active tabs. Most people posting about this have been upset by it, so I wonder whether there was a change in Beta/Developer/Nightly to reduce throttling?

You can check these throttle prefs on the about:config page.

• network.http.throttle.enable (true)
• network.http.throttle.version (currently 1 for me with beta)