Add firefox support

[andreicristianpetcu]

Most Chrome extensions work with no modifications in Firefox

https://vimeo.com/241435808

https://developer.mozilla.org/en-US/Add-ons/WebExtensions/Browser_support_for_JavaScript_APIs

[Ashish-Bansal]

@craftwar Are you interested in maintaining the Firefox version of this extension from official source code ?

Or if anyone else is interested, please let me know. I'll add its link to the wiki.

[craftwar]

@Ashish-Bansal I will maintain my version only. It almost does the same thing except audio picking order is reversed. I dunno which gets better audio quality.

[gxanshu]

@Ashish-Bansal i'm interested in maintain Firefox version.