I'm not very involved, but on top of the Chrome base API both Firefox and Edge have a bunch of additional APIs IIRC (I think Firefox supports all of the Chrome APIs but I'm not sure). The intent is to have the same manifest format so basic extensions work everywhere, but extensions can use browser-specific APIs that fit within the same general framework too.
> I think Firefox supports all of the Chrome APIs but I'm not sure
They support quite a few but not all. in addition there is a promise based version of each of the APIs that you can use in firefox under the browser namespace instead of the chrome namespace. (with a polyfill on npm too if you want that in chrome)
