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 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.