I have developed extension for chrome and later I made browser specific using gulp.

So I know left with folders.  

```
/Chrome
/Firefox
    |-/data/
    |-/locale/
    |-index.js
    |-package.json
/Safari

```

So for the chrome it was simple drag and drop.   

So problem I faced is when I try to load on firefox browser at [about:degugger](http://about:debugger) it would reject my extension saying it is corrupt.   

For Safari I searched on their website and I found documentation for only using Xcode editor.