Extensions add new functionality to Mozilla applications such as Firefox, SeaMonkey and Thunderbird. They can add anything from a toolbar button to a completely new feature. They allow the application to be customized to fit the personal needs of each user if they need additional features, while keeping the applications small to download.
Extensions are different from plugins, which help the browser display specific content like playing multimedia files. Extensions are also different from search plugins, which plug additional search engines in the search bar.
https://developer.mozilla.org/en-US/docs/Extensions
