1. Plugins, themes, add-ons and extensions are names that are confounded between browsers, eras, writers, etc.
2. Plugins is used predominately to describe software that bridge to or implement non-browser centric function. Java is a programming language used for many purposes besides browsing, and a plugin is used to integrate it with a browser.
3. Add-ons is a more generic term, which is often used to aggregate extensions, plugins, themes, etc.
4. Themes are appearance modifiers.
5. Extensions and add-ons are often used interchangeably. Firefox documents originally referred only to extensions and plugins. Themes came along, and then add-ons came into use to lump them all together.
6. Add-ons let you do more, do it faster, see it better, and otherwise make your browser what you'd like it to be.
7. Add-ons can increase the attack surface of any browser. All the browser developers are working to keep that from happening. They're not there yet.