Good addon, but too little sites are supported. Is there any way to add other sites (i.e. via custom js scripts)? And, BTW, is this addon open source? If so, where can I find it?
UPDATE:
After all the updates it is perfect!

Nifty little extension. Works out of the box even on linux! (FF42)

Works exactly as described, installed without needing a reboot and within a few seconds I could press the play/pause button and my most recent YouTube tab would pause. It keeps track of which YouTube tab is most recent in case you have more than one open and only toggles the playing on that one tab. No problems with it not working when Firefox is out of focus like others have reported. Using a Microsoft Natural Ergonomic Keyboard 4000 v1.0.

I can't seem to get it to work with my Microsoft Sidewinder X4 Keyboard. The keyboard uses "Microsoft Mouse and Keyboard Center" for the programming of media keys and macros. I'm running Firefox Developer Version 44.0a2 (2015-11-25)

chance to redefine or disable other multimedia keyboard keys in Firefox like Browser_home, Browser_refresh, Browser_favorites etc.? I am looking for such solution, but no extension provides this functionality.

tested using spotify web player and youtube

Nice add-on.
Is there any chance to make this work on OS X?

does exactly what you want and need it to do.

Quick easy install and my media button (pause) works on sites like Youtube! Signed up to Mozilla for this review :)

edit: Currently doesn't work at all for Firefox 42.0 :(

edit: Turns out the issue is IntelliType which is installed with "Microsoft Mouse and Keyboard Center" (so if you haven't rebinded any of your additional keys then best to just uninstall this because it doesn't do anything)

Works great, but it paused every time i went to another tab, but that was caused by another add-on now removed. Also works with the button on my mouse. Even the forward button works. only the stop button and back button dont work but what are they even supposed to do?
mayby the back button could be cool but thats diffrent than autoplay, that would mean you would have to go to the last page.

Fantastic, this works even when Firefox is not focused. Rather then adding multiple sites, maybe we can have a dynamic feature, where we can "train" it, so that pressing the media key triggers what we told it do, tricky I know :( But that way you don't have to add per site support, that must be exhausting

Switched from Chrome to Firefox not long ago and this is a feature I really used a lot (Chrome has quite a few plugins that enable media keys).

I only have one current chrome plugin installed for this functionality and that is:
Stream Keys: https://chrome.google.com/webstore/detail/streamkeys/ekpipjofdicppbepocohdlgenahaneen?hl=en

The other one was something generic like "Media Key Controller" or something like that. I will give you a link when I can.

Windows 10
Logitech G710+

I won't repeat myself but AWESOME!! :D

Nothing else that I've used in the past had been compatible on Linux, much less the range of applications supported by this add-on. To have a cross-platform media key plugin for Firefox is awesome.

[Disclaimer: have contributed code to this add-on]

Works great on YouTube with Logitech G15 v1 keyboard + Firefox v41.0.1 on Windows 10. This should be a default feature of Firefox!

I've been wanting this for a while. Works great!

Able to control YouTube when I'm on another tab or even when Firefox is not focused. (Keyboard - DasKeyboard 4 professional)

I don't know, why this addon has only 5 ratings. I'm using media keys for everything that supports them, but i never hoped that browser services will ever support this functionality. Thank for this!

Works great! I can even pause/skip Pandora while fullscreen in another application.

The extension seems to work on most web players with media key support, not just those mentioned. Works perfectly for Google Music, even when others designed for it no longer do.

Tested with elementaryos (ubuntu), working perfectly . Even when firefox is not focused.
Tested with soundcloud.

Simple, no restart and works exactly how it should. Only tried it with Pandora.