Firefox v57 notice:
CLEO is considered a 'legacy' extension and will no longer work with Firefox v57 and above. CLEO cannot be written with the WebExtensions platform that Mozilla has adopted for all extensions. CLEO will still be maintained, but only for Firefox clones (like Pale Moon).
CLEO 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.
You may have a list of your favorite extensions/themes that you would like to share with others. With CLEO, you can gather them all together into a single file (called, perhaps, "My favorite add-ons") and post it on a webpage or email it to family, friends and co-workers. They can install the whole batch with a just a couple of clicks! (e.g., File > Open File…)
Useful tool for developers: I, as an extension developer, have several different Firefox profiles. Whenever I start a new extension, I create a new profile for testing, debugging, etc. In this profile, I like to have a dozen or so developer related extensions installed. With CLEO, I can package all these extensions together and install them into a new profile with just a few clicks.
Once installed, CLEO is available from the Firefox Tools menu under the FEBE* submenu. (FEBE must be installed for CLEO to work.)
*Available elsewhere on this Mozilla site.
NOTE: If you are getting a message complaining about not having the correct version of FEBE installed (and you know the correct version is installed) see this post in the support forum.
(Please copy/paste the URL into your browser. This Mozilla site does not create links correctly when they contain ampersands.)