Summary : Firefox Extensions and Themes

What is a firefox add-on?


Add-ons add new functionality to Gecko-based applications such as Firefox, SeaMonkey, and Thunderbird.

There are two main types of add-on:  

1. Extensions add new features to the application
2. Themes modify the application's user interface.[UI]

Mozilla operates a repository at addons.mozilla.org, also known as AMO

You don't have to submit add-ons to AMO, but if you do, users can take confidence in the fact that they have been reviewed, and you can benefit from AMO's visibility as a source for useful add-ons.

As a developer one needs to follow a set of guidelines to help ensure that they provide a good experience to users.

There are three different techniques you can use to build extensions:

1. Add-on SDK-based extensions, 
2. Manually bootstrapped restartless extensions
3. Overlay extensions.

References:
https://developer.mozilla.org/en-US/Add-ons