Yes, you are 100% right and topic says it all that their is some thing that allows you to organize individual extensions/themes by combining them together into a single, installable file. This file can be used to install or restore a collection of extensions and/or themes.The name of this extension and geeks favi is CLEO.

What you can do with ClEO?
Every firefox users have its own list of useful and interesting extensions , with ClEO you may have a list of your favorite extensions/themes that you would like to share with others in a one pack.It patches all of your extension together into a single file.After you work every one can enjoy your fav extension for a single click ie anyone can install the whole batch with a just a couple of clicks! (e.g., File > Open File…)

Download to Firefox

Useful Link

Homepage for CLEO
Support forum for CLEO