Added Firefox hotkey support

Added support for hotkeys in Firefox

Play/Pause: All 4 media keys. Due to a bug in Firefox (I'm pretty sure) all 4 media keys correspond to Play/Pause. Support is listed here https://developer.mozilla.org/en-US/Add-ons/WebExtensions/manifest.json/commands#Browser_compatibility
Next Track: Due to the above bug, the Next Track command could not be bound to MediaNextTrack. I've bound it to Alt+N instead

I attempted to test this in Chrome as well and it did not seem to work as expected.

I have incremented the minor version number to 1.11.0

pvrs12 / Anesidora