Firefox: add-ons, extensions, and plugins

Most documentation uses these terms seemingly interchangeably, making it difficult to find good definitions. So I don't have to go on a wild Google chase again:

An add-on is an installable component for Firefox (or any Mozilla project). Extensions and plugins are two types of add-ons. An extension is an enhancement like a toolbar button or feature. A plug-in is a handler of a specific type of content (examples: Acrobat Reader, Flash).

A search plugin is different than a plugin. A search plugin plugs a search engine into the search bar. It may or may not be considered an add-on.