Mozilla is the name of the organization responsible for the Firefox browser. They had problems giving away their other web-browsers, so they created a brand new browser and set up some special advertising. They eventually named the new browser, "Firefox".
Some special terms exist for Mozilla browsers. Mozilla calls everything added, an addon. They then define two different types of addons: the extension and the plugin.
An Addon is an application added to Firefox. The word addon applies to both extensions and plugins.
Extensions are addons which extend the capabilities of the browser. They become a part of the browser and can be unloaded from the browser without going through a whole "Add/Remove Program" of the "Microsoft Control Panel" ritual. Compare that to plugins which require removal from the operating system.
Plugins get installed by an external operating system dependent application (installer) provided by companies other than Mozilla. The Microsoft Media Player represents the best example of such a plugin. Companies provide plugins to enhance (or cripple) the viewing of websites, documents or pages. Some examples of plugins include the Adobe Portable Document Reader, Windows Media Player and MacroMedia Flash Player. The plugin typically shows up as external software. On Windows systems, the files for plugins include DLLs and Executables. When a browser wants to show a document which gets run by a plugin, the browser makes a call to specific executable file (.exe or .dll) and then the external file renders the document inside the browser.
The best example of a plugin that ALWAYS cripples the viewing of any document equates to Adobe's Adobe Portable Document (pdf) Reader. If you want to publish a crippled document, publish in a PDF format. Adobe cripples YOUR FREEDOM to browse through and read the information contained within it. Compare a pdf file to an html website or a set of html pages. Why must you always blow a .pdf file up to 125% and still get fuzzy characters that are hard to read? Absolutely ridiculous!
The Official Plugin Link For Mozilla Firefox
The Web Developer Extension: http://chrispederick.com/work/webdeveloper/
The No-Script Tool: http://www.noscript.net/whats