Whatever window you are currently in will have the focus.

I don't believe that Firefox has native support for media keys at the moment. However, the Media Keys provides that functionality.

The limitations are that it only works on select websites. Also, it doesn't appear to be able to control tabs that are not focused. However, that's understandable since it would be impossible to determine which video you are trying to control if, for example, you have two YouTube tabs open.

Hope this helps.

In addition to the above :

https://addons.mozilla.org/en-US/firefox/addon/play-pause/

But please first read what the developer says under "About this extension".

The reviews are very good though .....

Also see : https://www.ghacks.net/2017/08/01/pause-toggle-firefox-tabs-html5-media/

Wesley Branton said

I don't believe that Firefox has native support for media keys at the moment. However, the Media Keys provides that functionality. The limitations are that it only works on select websites. Also, it doesn't appear to be able to control tabs that are not focused. However, that's understandable since it would be impossible to determine which video you are trying to control if, for example, you have two YouTube tabs open. Hope this helps.

Chrome can do it especially when chrome is not being used. I could be playing a game in fullscreen and still be able to pause a video

That's strange because I tested with Google Chrome when I first replied and I couldn't get my media keys to work there, even for focused tabs.

Regardless, Firefox and Chrome are entirely seperate browsers. Firefox does not currently have builtin media key support, unfortunately.

Wesley Branton said

That's strange because I tested with Google Chrome when I first replied and I couldn't get my media keys to work there, even for focused tabs. Regardless, Firefox and Chrome are entirely seperate browsers. Firefox does not currently have builtin media key support, unfortunately.

With chrome you have to edit some setting in the code if I recall.

Maybe you could contact the developer of Streamkeys and ask him to make his extension available for Firefox :

Email address : alex@aegabriel.com

Unless of course that extension is not what you would want to use ....

Maybe you'll find something useful in this Reddit thread :

https://www.reddit.com/r/firefox/comments/ajhzfz/media_keys_for_youtube/

(edit : typo)