Add-ons are small pieces of software which change or add to the appearance or functionality of Firefox. There are three kinds of add-ons: extensions, themes, and plugins.
Extensions: Extensions add new functionality to Firefox. 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 while minimizing the size and appearance of the application itself.
Themes: Themes modify the appearance of Firefox and allow you to personalize it to your taste. A theme can simply change the appearance of toolbar buttons, or it can completely change the look and feel of Firefox.
Plugins: Plugins help Firefox perform specific functions like viewing special graphic formats or playing multimedia files. Plugins are slightly different from extensions, which modify or add to existing functionality. A typical purpose of a plugin is to add multimedia capabilities, such as the ability to embed and play videos on a web page.